Cauliflower and Caramelized Onion Tart

Name: Cauliflower and Caramelized Onion Tart
Ingredients: 1 small head of cauliflower (about 1 pound), cored, cut into 1-inch florets; 2 1/2 tablespoons olive oil, divided; 1 tablespoon truffle oil; 1 refrigerated pie crust; 1 large onion, halved lengthwise, thinly sliced; 1 tablespoon Dijon mustard; 2 large eggs; 1 7- to 8-ounce container mascarpone cheese (Italian cream cheese); 1 /2 cup whipping cream; 1 /4 teaspoon ground white pepper; Pinch of ground nutmeg; 1 cup grated Gruyère cheese; 3 /4 cup grated Parmesan cheese