Fresh black truffles are worth the splurge. They are available at some specialty foods stores and from Urbani Truffles (215-699-8780; urbanitruffles.com). Black truffles in jars are sold at specialty foods stores and Italian markets; the most flavorful are European winter black truffles.
Ingredients
Makes 10 servings
4 cups whipping cream, divided
2 tablespoons chopped fresh thyme
15 whole black peppercorns
5 large garlic cloves, crushed, divided
5 pounds long slender russet potatoes, peeled, cut into 1/8-inch-thick slices
3 fresh black truffles or three 1/3-ounce jars whole black truffles, sliced paper-thin
Step 1
Combine 2 cups cream, chopped thyme, black peppercorns, and 3 garlic cloves in heavy medium saucepan. Bring to simmer over medium heat. Cover, reduce heat to medium-low, and simmer 15 minutes. Add remaining 2 garlic cloves. Cover; simmer 5 minutes. Strain cream into bowl.
Step 2
Preheat oven to 400°F. Butter 13x9x2-inch glass baking dish. Arrange 1/4 of potatoes in even layer in dish. Sprinkle with salt and pepper. Scatter 1/4 of truffle slices over. Spoon 1/4 of cream over. Repeat 3 more times, sprinkling any truffle liquid from jars over potato layers. Pour 2 cups cream over. Press potatoes firmly to compact. Cover dish with foil.
Step 3
Bake gratin 1 hour. Uncover and bake until top is brown, potatoes are tender, and cream bubbles thickly, about 20 minutes longer. Let stand 10 minutes before serving.