Cauliflower, Caramelized Onion, Gruyere and Truffle Tart (i.e. The Most Ridiculous Tart Ever)

This tart is amazing - if you are a fan of cheese, caramelized onions, or truffles, you have to make this.  It's so rich that you'll probably only be able to have one piece, but you'll probably end of up eating more anyway.  Adapted from Smitten Kitchen.

Ingredients

1 small head cauliflower, chopped into small florets
3.5 T olive oil
1/2 T truffle oil
1 refrigerated pie crust (you can make your own if you want, see this recipe)
2 large onions, thinly sliced
1 T dijon mustard
2 eggs
1 container (7-8oz) mascarpone cheese
1/2 c full-fat milk (or cream)
1/4 t pepper
pinch of ground nutmeg
1 c grated gruyere cheese
1/3 c grated parmesian cheese

Instructions

1) Preheat oven to 425F.  Toss cauliflower in 2 T olive oil, spread on a baking sheet, sprinkle with salt and pepper, and roast 25-30 minutes, tossing them halfway through to avoid burning.
2) Toss cauliflower with truffle oil and lower oven temperature to 350F.
3) Press pie crust into the bottom of a 9 inch tart pan, cover with foil, and pre-bake for 20 minutes, until the crust is golden.  (Use foil and pie weights if you have them.)  Let cool.
4) Heat remaining 1.5 T olive oil in large skillet over medium heat.  Add onion, sprinkle with salt and pepper, and cook until a deep gold brown, stirring occasionally, 30-40 minutes.
5) Whisk together eggs, mascarpone, milk, and nutmeg in a large bowl, then add the gruyere.
6) Spread dijon mustard over the bottom of the tart pan, then sprinkle with onions.
7) Arrange cauliflower on top of mustard and onions, then pour cheese mixture on top.
8) Sprinkle with parmesian, and bake until the tart is gold and the center i set, about 40 minutes.  Cool 10-15 minutes before serving.