Cauliflower and Carmelized Onion Tart

1. Position rack in center of oven and preheat oven to 425°F. Toss cauliflower with 2 tablespoons olive oil in large bowl. Spread on rimmed baking sheet, sprinkle with salt and pepper and roast 15 minutes before turning florets over and roasting until brown and tender, another 15 minutes in my oven, 25 minutes according to the original recipe. Cool cauliflower then thinly slice (a direction I entirely missed when originally make this; I left my florets in chunks and enjoyed it that way) and drizzle with truffle oil or sprinkle with truffle salt, if using. (See notes below about these ingredients.) Reduce temperature to 350°F.
2. If using store bought pie crust, press it onto the bottom and up the sides of a 9-inch diameter tart pan with removable bottom. Line crust with foil, fill with pie weights and bake 20 minutes. Remove foil and weights then bake until crust is golden, about 5 additionally minutes. Press crust back with the back of a fork if bubbles form. Cool crust and maintain oven temperature.
3. [When using the homemade tart crust (recipe below), I do not find that it needs to be par-baked. It is thin and rather dry so it bakes up pretty crisp. Store bought pie doughs are a little bit softer, so the par-baking keeps it from getting soggy.]
4. Heat remaining 1 1/2 tablespoon olive oil in a heavy large skillet over medium-low heat. Add onion, sprinkle with salt and pepper and cook until onion is a deep golden brown, stirring occasionally. This took me just shy of 30 minutes, though the original recipe suggests 40. Cool slightly.
5. Use a knife or brush to spread the bottom and sides of crust with mustard. Spread onion over crust. Arrange cauliflower over the onion. Set the tart on a rimmed baking sheet (to protect against leaks). Whisk eggs, mascarpone, cream and pepper in a 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 and cool 15 minutes before serving.
6. Do ahead: Onions can be caramelized, dough can be parbaked (or rolled and pressed into pan, if homemade) and cauliflower can be roasted a day ahead. Store a parbaked crust at room temperature, a rolled-out unbaked crust and cauliflower and onion in the fridge. Cauliflower and onion should be kept in separate containers. Whole tart can be made and baked a day in advance, reheated in a low oven before serving.

A Great Savory Tart Shell

1. In a large bowl, combine the flour, cornstarch and one-fourth teaspoon salt. Cut the butter in with a pastry blender, fork or two knives until it is in very tiny bits. Add one egg and mix with a fork until a dough forms. If this does not happen easily, toss it out onto a counter and knead it together. This dough is rather tough but with a little elbow grease, it does come together nicely. (Dough can also be made in a food processor, or as the original recipe suggests, in a stand mixer, though I have not tested in in the latter.)
2. On a lightly floured surface, roll the dough out to a 12-inch circle. Place the dough in a 9-inch pie plate or tart pan and press to remove any air bubbles. Crimp the edges, and refrigerate for 30 minutes. Proceed with a filling of your choice, no parbaking required.