Celery Root (Celeriac) and Potato Gratin

Scalloped potatoes with ‘tude

Easter arrives in a few days, so menu planning is ramping up. Having a big roast? Or maybe baked ham or a leg of lamb? If you’re serving any of the above, you’ll probably want a rich, starchy side dish to go with it.

Scalloped potatoes are a popular choice, of course—and this dish takes them as a starting point. But then we add celery root (celeriac) to dial the flavor up a notch. We also add some sharp Gruyère and Parmigiano-Reggiano cheese, and just a touch of zippy Dijon mustard.

The result is irresistibly good. But fair warning: If you’re feeding a holiday crowd, you may want to double this recipe. Otherwise, the serving dish might not make it all the way around the table.

Recipe: Celery Root (Celeriac) and Potato Gratin

You can omit the potato entirely and just use celery root (celeriac) in this dish. But celery root tends to be pricey in the US (where it’s not widely used). Besides, the flavors of celery root and potato marry beautifully. So I often combine them just because they bring out the best in each other.

For a gratin like this one, my basic rule of thumb is to use a cup of liquid (cream, half-and-half, or milk) and 4 or 5 ounces of grated cheese for every pound of potatoes/celery root. You can vary these amounts a bit—this is a pretty forgiving recipe—but this formula always works well for me. It’s the same ratio we used when making the Gratin Dauphinois that we discussed a couple of years ago.

This recipe makes enough to fill a gratin dish that measures about 8 x 11 inches (13 inches with handles) and that holds about 7 cups of liquid. This will be enough for 6 to 8 servings. But your guests will want seconds, so you if you’re feeding a crowd, you really might want to double the recipe. In that case, use a casserole that measures about 9 x 13 inches (a Pyrex dish is ideal).

Prep time for this recipe is about 15 minutes (it could be 5 minutes more or less depending on how quickly you work). Then add another hour or so (mostly unattended) for baking time.

Leftovers keep well for a few days if refrigerated in an air-tight container.

Ingredients

• 2 medium shallots, minced (perhaps 4 tablespoons; exact amount not critical)
• 1 tablespoon butter
• 2 cups half-and-half, milk, cream, or a mixture (I prefer half-and-half; see Notes)
• 1 tablespoon Dijon-style mustard
• 1 teaspoon kosher salt (or to taste)
• ¼ teaspoon freshly ground black pepper (or to taste)
• ~8 ounces grated Gruyère (or another hard Swiss cheese; about 2 - 3 cups)
• ~2 ounces grated Parmigiano-Reggiano (about 1 cup)
• ~1 pound celery root (1 large or two medium-sized)
• ~1 pound potatoes (I prefer russets, but waxy ones work well; see Notes)
• additional butter for greasing baking dish (maybe a couple of teaspoons)
• parsley or chives for garnish (optional)

Procedure

1. Preheat oven to 375 degrees Fahrenheit. Arrange one rack in the middle of the oven, and another in the bottom position. On the lower rack, place a baking sheet, preferably one that’s rimmed. The purpose of this is to catch any drips if the liquid in the gratin bubbles over while baking (this doesn’t usually happen, but it can). You may wish to line the baking sheet with aluminum foil for easier cleanup.
2. Peel the shallots and mince them finely.
3. Place a 2-quart saucepan on medium stovetop heat. Add 1 tablespoon butter, and when it’s melted and sizzling, add the minced shallots. Sauté for 4 minutes.
4. Add the half-and-half (or cream, or whatever you’re using), mustard, 1 teaspoon kosher salt, and ¼ teaspoon freshly ground black pepper to the saucepan. Heat until the half-and-half is warm (you don’t want to bring this to a simmer, just warm it well).
5. Meanwhile, grate the cheeses and set them aside. I suggest using the grating attachment on the food processor to speed things along, but a 4-sided box grater works fine too.
6. Scrub and peel the celery root and potatoes (see Notes for instructions on how to peel celeriac; it discolors once it’s peeled, but that doesn’t matter much for this dish). Cut the celery root and potatoes into thin slices (about 3/16 inch is ideal; see Notes). You can use a mandoline, a vegetable slicer, or a knife for slicing. If you’re using a mandoline, remember that the blade is wicked sharp—use the safety guard that comes with it so you don’t risk losing a finger.
7. Butter the baking dish well.
8. By now, the half-and-half should be warm, so you can begin to assemble the dish. Arrange some celery root slices in the baking dish so they overlap and form a layer. When the first layer is complete, sprinkle with Gruyère and add a small ladle of the warm half-and-half. Repeat with a layer of potatoes. Continue until you’ve used all the celery root and potatoes, saving perhaps a quarter of the Gruyère for the top layer. Once you’ve layered all the celery root and potatoes, pour on the remaining half-and-half. Top the gratin with Gruyère and sprinkle on the Parmesan, covering the top evenly.
9. Bake the gratin on the middle oven rack until done, usually about an hour. “Done” means the top is a nice, bubbly brown—and if you insert a paring knife into the potatoes, it goes in without resistance. If the potatoes are cooked through, but the top of the gratin isn’t as brown as you’d like, run it under the broiler for a minute or two.
10. Garnish the gratin with parsley or chives (if desired), and serve.

Notes

• I like the flavor of shallots, so I sometimes double the amount specified here. You can reduce or omit the shallots if you want—the dish will still be pretty good, though it will lack a little ting.
• Cream is the traditional liquid of choice for a gratin like this. I think half-and-half tastes just as good, without quite as much fat. You can also use milk (even skim milk), although the flavor won’t be as luscious. Whatever your choice of dairy, it will reduce as it cooks—making a nice, thick sauce.
• The starch in potatoes thickens half-and-half, which is why you wouldn’t want to use béchamel sauce (which is already thick) when making this dish.
• One tablespoon of mustard provides subtle flavor in this dish. If you want a more in-your-face mustard presence, you can increase the amount to 2 tablespoons.
• You can also add a bit of hot sauce (such as Tabasco) if you want added zip.
• I like to use equal parts of celery root and potato for this dish. But if you have a bit more of one than the other, it won’t matter.
• Celery root has a tough hide, but a good swivel peeler works quite well or it. Or do as I do: Perch the celeriac on one end, then use a knife to slice off the peel (much as you might cut the peel off an orange). You lose a bit of the celeriac flesh this way, but you’ll be able to peel it in well under a minute.
• Waxy potatoes are traditional in most potato gratins because they hold their shape when cooked. I like to use waxy varieties when I’m slicing potatoes rather thickly (¼ inch or a bit more, as I sometimes do for classic scalloped potatoes). But when I’m slicing them thinly—3/16 inch or less, as in this dish—I prefer to use russet (baking) potatoes. I find that the slices merge with the cheese and half-and-half, and “melt” together.
• If you’d like to turn this recipe into a hearty one-dish meal, just mix in some pieces of cooked ham or bacon.

Veggie Underground

“Wowzer,” said Mrs. Kitchen Riffs, taking a bite. “I thought your Gratin Dauphinois couldn’t be beat, but this is even better.”

“The celeriac bumps up the flavor,” I said. “It somehow makes the potatoes taste even more potato-y.”

“Yes, and the Dijon mustard adds intrigue too,” said Mrs K R. “The flavor is really subtle, but you know something is going on.”

“Mustard and celery root make such a natural combo,” I said. “Remember, we used mustard in the dressing for the Celery Root Rémoulade we posted about in February."

“Another wonderful dish,” said Mrs K R. “One of the best you’ve done all year. And one that would fit well with Easter menus, too.”

“It’s a shame that celeriac doesn’t get the love it deserves,” I said. “Maybe because it grows underground.”

“It also looks like a cross between a rock and a giant turnip,” said Mrs K R.

“True, but beauty isn’t everything,” I said. “Maybe it just needs a good PR agent. One who can come up with a memorable tag line for it.”

“Hey, I’ve got it: The Undercover Veggie—Taking Celery Back to its Roots," said Mrs K R.

You’ll be glad to hear that Mrs K R has no plans to start a PR firm anytime soon.

You may also enjoy reading about:
Gratin Dauphinois
Celery Root (Celeriac) Rémoulade
Braised Celery with Tomato and Parmesan
Italian Celery and Mushroom Salad
Roast Asparagus
Hollandaise Sauce
Or check out the index for more recipes