The key to this Root Vegetable Gratin with Mustard Cream is having all the vegetables sliced the same thickness so they cook at the same rate. Make friends with a mandoline: It quickly yields precise, even slices.
- unsalted butter (as needed for baking dish)
- 2 teaspoon whole yellow mustard seeds (divided)
- 2 pound assorted root vegetables (such as sweet potatoes, celery root, and rutabega) (peeled and very thinly sliced)
- 2 clove peeled, whole garlic
- 2 cup whole milk
- 2 cup water
- 1 cup cream
- 1 tablespoon Dijon mustard
- kosher salt (to taste)
- ground white pepper (to taste)
- 2 tablespoon all-purpose flour
- 6 ounce grated Emmental cheese (or similar)
Preheat oven to 375 degrees F.
Butter the sides and bottom of a 10″ x 7″ inch oval baking dish.
Using a mortar and pestle, grind 1 teaspoon of the mustard seeds into a very coarse powder. Set aside.
In a large saucepan, combine sliced root vegetables, garlic, milk, and water. Bring the mixture to a boil, then reduce the heat to medium-low. Simmer about 5 minutes until the vegetables begin to soften; drain, saving 1‑cup of cooking liquid. Discard garlic; set vegetables aside.
Combine the reserved cooking liquid with the cream. Whisk in the Dijon mustard and the roughly ground mustard seeds. Season with salt and white pepper.
Layer the softened vegetables in the baking dish, sprinkling a little flour and some of the cheese between each layer. Reserve half of the cheese for the top. Slowly pour the mustard-cream mixture over the vegetables until it comes almost to the top of the dish. Sprinkle with the remaining cheese and distribute the remaining whole mustard seeds evenly across the top. Place the baking dish on a cookie sheet to catch the bubble over. Bake in the heated oven for 25 to 30 minutes, or until browned and bubbly. Let the gratin rest about 5 minutes before serving.