Creamy white cauliflower is available year round, and this rich, elegant tart certainly puts a new spin on a vegetable that's often unappreciated. Cauliflower is just sumptuous when roasted till golden brown and crispy, then combined with sweet caramelized onions in a rich, cheesy custard.
We like Flaky Multigrain Pie Crust for this tart, or use your favorite recipe.
The cauliflower and onions can be prepared up to 2 days ahead of assembling the tart, and the pastry crust can be prebaked early in the day for filling and baking later. The finished tart, however, doesn't keep well; if it sits too long, the crust gets soggy (even with blind baking), so enjoy this delicious dish the day it's made.
Position a rack in the middle of the oven and preheat to 425 degrees F.
Place the cauliflower florets in a large bowl, add the truffle oil (or olive oil) and toss to coat. Transfer the florets to a rimmed baking sheet, arranging them so they don't touch one another, and season generously with sea salt and black pepper. Roast for 15 minutes. Then flip the florets with a spatula and continue cooking until the cauliflower is tender and browned, 15 to 20 minutes more. Set the florets aside to cool.
Reduce the oven temperature to 375 degrees F.
Remove the Multigrain Pie Crust dough from the refrigerator and open the parchment paper to a flat rectangle. Let the dough sit at room temperature for 10 to 20 minutes to soften. If the dough is too cold or hard, it will crack when you try to roll it out. (If you're using your own dough recipe, this may not be necessary.)
Lightly dust a rolling pin and the top surface of the dough with flour. Roll the dough into an 11-inch round and transfer it to a 9-inch tart pan with a removable bottom. Pat the dough into the pan and up the sides, then roll the pin around the edges to trim off any excess. Brush off any excess flour with a pastry brush. If there are holes or cracks, press the dough back together or patch them with small bits of extra dough. Line the tart shell with parchment, and fill it with pie weights or dried beans. Place the tart pan on a rimmed baking sheet and bake for 20 minutes.
Remove the baking sheet from the oven and lift out the paper and pie weights. Lightly prick the bottom of the pastry with a fork and return the sheet to the oven. Bake the pastry until it dries slightly and begins to take on a faint color, 8 to 10 minutes.
Remove the baking sheet from the oven, set the tart pan on a wire rack and allow the crust to cool slightly, about 10 minutes. Then carefully brush the bottom and sides of the pastry shell with the mustard. Set aside at room temperature.
While the crust is baking, cook the onions. Heat a large skillet, preferably cast iron, over medium-low heat. When it's warm, add the olive oil. Add the onion slices and cook, stirring frequently, until they soften and caramelize, 30 to 40 minutes. Be patient and do this slowly; the onions should be golden, not brown.
While the onions are cooking, thinly slice the roasted cauliflower florets and set them aside.
Transfer the caramelized onions to the prebaked tart shell and spread them out in a thin layer. Top with the sliced roasted cauliflower. Transfer the tart pan to a rimmed baking sheet.
Combine the eggs, mascarpone, cream, white pepper and nutmeg in a medium-size bowl and whisk to combine. Stir in the Gruyère. Pour the mixture over the cauliflower and onions in the tart shell and sprinkle the Parmesan over the top. Bake until the tart is puffed, set in the center and golden brown, 30 to 40 minutes.
Transfer the tart to a wire rack and let cool for at least 15 minutes before serving.
Per serving: Calories 610, Fat 53 grams, Cholesterol 165 mg, Sodium 380 mg, Carbohydrates 23 grams, Fiber 2 grams, Sugars 5 grams, Protein 15 grams.
Percentage Daily Values per serving (based on a 2,000-calorie diet): Vitamin A 25%, Vitamin C 45%, Calcium 30%, Iron 4%.