Cauliflower and Caramelized Onion Tart

Ingredients

• 1 small head cauliflower cut into 1-inch florets
• 3½ tablespoons olive oil
• 1 teaspoon truffle oil optional
• 1 tart shell
• 1 large onion halved lengthwise and sliced thin
• 1 tablespoon dijon mustard
• 2 large eggs
• 8 ounces mascarpone or a bit less
• ½ cup whipping cream (or low or full fat milk or cream)
• ¼ teaspoon ground pepper white or black
• nutmeg pinch
• 1 cup gruyere cheese grated
• ⅓ cup parmesan cheese
• A Great Savory Tart Shell
• 1¼ cups flour
• 1 tablespoon cornstarch plus 2 teaspoons
• ¼ teaspoon salt
• 6 tablespoons butter diced
• 1 egg