Step 1

Position rack in center of oven; preheat to 425°F. Toss cauliflower with 1 tablespoon olive oil in large bowl. Spread on large rimmed baking sheet, spacing apart. Sprinkle with salt and pepper. Roast 15 minutes; turn florets over. Continue roasting until tender, about 25 minutes longer. Cool cauliflower, then thinly slice. Drizzle with truffle oil; toss. Reduce oven temperature to 350°F.

Step 2

Press pie crust onto bottom and up sides of 9-inch-diameter tart pan with removable bottom. Line pie crust with foil; fill with pie weights. Bake crust 20 minutes. Remove foil and pie weights; bake until crust is golden, about 5 minutes, pressing crust with back of fork if bubbles form. Cool crust. Maintain oven temperature.

Step 3

Heat remaining 1 1/2 tablespoons olive oil in heavy large skillet over medium heat. Add onion; sprinkle with salt and pepper. Cook until onion is deep golden brown, stirring occasionally, about 40 minutes. Cool slightly. DO AHEAD Can be made 1 day ahead. Store crust at room temperature. Cover and chill cauliflower and onion separately.

Step 4

Brush bottom and sides of crust with mustard. Spread onion in crust. Arrange cauliflower evenly over. Set tart on rimmed baking sheet. Whisk eggs and next 4 ingredients in medium bowl. Stir in Gruyère. Pour mixture over filling in tart pan; sprinkle with Parmesan. Bake until tart is golden and center is set, about 40 minutes. Transfer to rack; cool 15 minutes before serving.