Caramelized Onion Tart

Last weekend, my sister, my mom and I decided to take a French cooking class. There was free wine, delicious food, and lots of cheese. Yes, it was every bit as amazing as it sounds. We learned how to make an array of things, like Leeks & Mushrooms a la Grecque, New York Strip Steak with Horseradish Crust, Pommes Dauphinoise, and my personal favorite... a Caramelized Onion Tart. Believe me when I say that this tart is out of this world. It's got flavor like you wouldn't believe, and I would honestly eat it every day if I could. One of the best things about it is that it can be made a day ahead. In fact, you're encouraged to do so! It involves making a Pate Brisee, (pastry dough) for the crust, which I will provide the recipe for. So, without further ado...
For The Pate Brisee:

• 1 pound butter, out of the refrigerator for about an hour
• 2.5 pounds flour
• 1 egg plus enough water to equal 1 cup

1. Firstly, you're going to want to cut your butter. Chef taught us an awesome trick for this: cut the block of butter into thin slices, but sprinkle flour in between each cut. This does WONDERS for making sure the butter doesn't clump together too much. After that, slice in the other direction to create small cubes. 
2. Press the butter into the rest of the flour until the mixture resembles small crumbs. A few pulses of the food processor will save you some time with this step.
3. Place the egg in a measuring cup, then fill it up will water until it reaches 1 cup. Lightly whisk until combined, then pour into butter mixture and begin to knead.
4. As soon as the dough begins to take shape, remove from the bowl and wrap in plastic wrap. It is SUPER important not to overwork the dough... just a few turns should do it. Allow to rest for 30 minutes in the refrigerator before using.

Makes 6 - 11"tart shells. You can freeze the dough for up to 3 months.
For The Tart:

• 8 ounces Pate Brisee
• 4 Large Spanish onions
• 2 ounces butter
• 1 egg
• 1/2 cup heavy cream
• 1/2 cup Parmesean cheese

1. Roll the dough out to an 1/8 inch thick circle larger than an 11 inch tart pan. Place the dough in the tart pan and blind bake in a 350 degree oven for 15-20 minutes, or until evenly golden. Do not trim the extra dough! Wait until the crust is done, otherwise you'll have to deal with shrinkage, which isn't fun. If you're not familiar with blind baking, it's pretty simple. All you do is put some parchment paper over the dough and fill it with pie weights (or some dried beans) before you bake it. This ensures that the crust doesn't puff up too much. 
2. Once the crust is done, allow it to cool, then trim the edges with a serrated knife. In a large pan, sweat the onions over medium heat with the butter, and cover with a lid. When the onions become clear and soft, remove the lid, and turn up the heat to high. Stir constantly while the onions begin to caramelize. Once the onions are a deep brown color, remove them from the heat and allow them to cool.
3. Combine the egg and cream in a small bowl, then mix with the cooled onions. Pour the mixture into your cooled crust, and evenly distribute. Sprinkle the Parmesean cheese on top, and bake in a 350 degree oven until the tart is set, about 20-30 minutes. Allow to cool before cutting.