A flavorful, savory gratin made with Yukon gold potatoes and celeriac and topped with Gruyere.

2 medium-sized celeriac (celery root)

5 Yukon Gold Potatoes (or an all-purpose potato)

4 cups half-and-half (or 3 cups heavy cream + 1 cup whole milk)

1½ cups shredded Gruyere

Fresh or dried thyme

1½ teaspoons salt

¼ teaspoon fresh ground black pepper

1 Tablespoon butter, for baking dish

1 garlic clove, cut in half

Preheat oven to 350°F.
To prepare the celeriac - cut off both ends of the celeriac. Use a sharp paring knife to peel the tough outer skin. Just as you would with a potato, remove any dark pits or "eyes". Soak the celeriac in acidulated water--use 1-2 Tbsp. fresh lemon juice or white vinegar. The celeriac brown quickly when exposed to air. Cut each celeriac into ¼" slices. Place the sliced celeriac back in the acidulated water.
Peel and slice the potatoes into ¼" slices.
Place the sliced celeriac and potatoes in a large stockpot. Pour the half-and-half (or milk and cream) over the sliced vegetables. Add a good amount of freshly ground black pepper (1/2 tsp.) and salt (about 1-1/2 tsp.) Bring to a simmer over medium heat. Cook until the tip of a knife goes through the slices with a little resistance. (You do not want to cook the potatoes and celeriac until they are completely cooked.)
In the meantime, rub the inside of a 9" by 13" glass pan with the garlic halves. Grease the bottom and sides of the pan with a generous amount of butter. Pour the partially cooked vegetables into the prepared pan. Sprinkle the top with the fresh or dried thyme. (I used dry because I was out of fresh--use a little less.) Top with grated Gruyere cheese. Bake for about 30-45 minutes or until the top is brown and bubbly. Let cool slightly before serving.
Serve with roasted or broiled meats, like steak or pork tenderloin.