This gratin, adapted from Deborah Madison's Vegetarian Cooking for Everyone, is lovely: the potatoes are velvety on the tongue and the celeriac adds a slightly mysterious note, making the dish feel a little more sophisticated than your standard potato gratin.
• 1 garlic clove
• 1 celery root, about 1 pound, scrubbed
• 1 pound potatoes (Yellow Fin or Yukon Gold)
• 1⁄2 cup heavy cream
• 2 tsp Dijon mustard
• salt and pepper
• 1 cup grated Gruyère
Preheat the oven to 375 F.
Cut the garlic clove in half, and rub a nine- by 11-inch gratin dish with the cut side, then butter the dish.
Peel the celery root, and put the trimmings in a pan with what's left of the garlic and three cups of water. Set a steamer over the top, and bring to a boil. Quarter the celery root, then slice it 1⁄4 inch thick.
Peel the potatoes and slice them into 1⁄4-inch rounds. Put the potatoes and celery root in the steamer and steam for five minutes, or until tender. Remove to a large bowl.
Strain the cooking liquid and reserve 11⁄4 cups. Mix the liquid with the cream and mustard, and add salt and pepper to taste. Pour the mixture over the vegetables and toss well. Transfer the vegetables to the gratin dish, smooth them out, and cover them with the Gruyère. Bake until bubbling and browned on top, about 30 minutes.
To read Anastatia Curley's post about turning an ugly root into this gratin, click here for the story.