Potato Gratin with Gruyère, Thyme & Horseradish

I’ve been making this creamy gratin for holiday dinners for years — it’s my go-to side for roast beef. Thinly sliced Yukon gold potatoes, a mix of cream and chicken broth, plenty of grated (best quality) gruyère cheese and some chopped fresh thyme come together to make this sing. The addition of a bit of horseradish or Dijon mustard can give it a subtle kick, but you can also leave it out if you like.

Whenever possible, I cook my gratins in shallow, enameled cast iron gratin dishes. (Le Creuset makes my favorite.) But a 2-quart (7 x 11”) Pyrex baking dish will work fine here, too. Just remember you want a shallow, not a deep dish.

For the best texture, I don’t pre-cook my potatoes before putting them in a gratin – I simply slice them very thinly so they’ll cook through in the oven. The easiest (and safest) way to do this is by cutting the potato in half lengthwise first, turning it cut-side down on the cutting board, and slicing across with a thin-bladed knife like a Santuko or ceramic knife.

Serves 4 to 6

• 1 teaspoon unsalted butter
• 1 (generous) cup fresh breadcrumbs
• 1 tablespoon extra-virgin olive oil
• 2 teaspoons roughly chopped thyme leaves
• 3 tablespoons coarsely grated Parmigiano (I like to chop the Parmigiano in a small food processor to get pieces the size of very small pebbles)
• Kosher salt
• 1 cup plus 2 tablespoons heavy cream 
• 3/4 cup low sodium chicken broth
• 2 teaspoons prepared horseradish or 1 ½ teaspoons Dijon mustard
• 1½ pounds Yukon Gold potatoes (about 6 medium-small), peeled
• Freshly ground pepper
• 1 generous cup (2 ½ ounces) coarsely grated gruyère

1. Heat the oven to 350°F. Rub a 2-quart shallow gratin dish with the butter.

2. In a small bowl, combine the breadcrumbs with the olive oil, a big pinch of salt, ½ teaspoon of the thyme, and the Parmigiano.

3. In a liquid measure, combine the cream, the broth, and the horseradish. Mix well.

4. Cut the potatoes in half lengthwise and turn the halves cut-side down on a cutting board. With a sharp knife, slice the halves across as thinly as you can so that you have thin half-moon-shaped pieces.

5. Put the potatoes in a mixing bowl. Add 1 teaspoon salt, several grinds of fresh pepper, the cheese, the remaining thyme, and the cream mixture. Mix well. Transfer all to the prepared gratin dish.

6. Scooch the potatoes around until they are as evenly dispersed as possible. Using your palms, press down on the potatoes to bring the liquids up and around them as much as possible. (It won’t necessarily completely cover them.) Cover the top with the breadcrumb mixture.

7. Bake until the potatoes are tender when pierced with a fork (check the middle of the dish as well as the sides), the breadcrumbs are brown, and the juices around the edges of the gratin have bubbled down and formed a dark brown rim around the edge, 65 to 70 minutes. Let cool for about 15 minutes before serving.