cauliflower and caramelized onion tart

INGREDIENTS

• 1 small head of cauliflower (about 1 pound) or 1 pound of a larger head of cauliflower, cut into 1-inch flowerets (Romanesco cauliflower, especially orange or green, would be a pretty substitute)
• 3 1/2 tablespoons olive oil
• 1 tablespoon truffle oil or a few pinches of truffle salt (optional) (see Notes below)
• 1 refrigerated pie crust or a homemade tart shell (recipe below)
• 1 large onion, halved lenghtwise and thinly sliced
• 1 tablespoon Dijon mustard
• 2 large eggs
• 1 (7- to 8- ounce) container mascarpone cheese (see Note below for suggested substitutions)
• 1/2 cup whipping cream (although any low- or full-fat milk or light cream will work as well)
• 1/4 teaspoon ground white or black pepper
• Pinch of ground nutmeg
• 1 cup grated Gruyère cheese (Swiss or Comté are great swaps)
• 1/3 cup Parmesan cheese
• 1 1/4 cups flour
• 1 tablespoon plus 2 teaspoons cornstarch
• 1/4 teaspoon salt
• 6 tablespoons butter, diced
• 1 egg