Make the Pie Crust:

1. Add the flour and the butter to a large bowl. Toss in the cold butter pieces and coat with flour. Using your fingertips, start working the butter into the flour. Continue until the butter is broken down into small pieces, no bigger than the size of a pea. You can also do this with a pastry blender.
2. Drip the ice water around the edge of the bowl, and use a rubber spatula to make sure the dough is largely hydrated. If there is enough water the dough should easily squeeze together with your hands. If needed, you can drip in more water into any dry spots (especially check the bottom of the pile), just enough to bring the dough together.
3. Gather the dough together and wrap it tightly in plastic wrap. Using a rolling pin, roll the wrapped dough out until it stretches to the corner of the plastic wrap. Refrigerate for at least 2 hours, or up to two days.

Make the Caramelized Onions:

1. Heat oil in a wide, heavy-bottomed pan on medium heat. Add onions and cook for 10 minutes, stirring a few times to make sure all of the onions contact the pan and start to brown slightly.
2. Add a pinch of salt and stir to combine. Lower the heat to medium-low and let onions sit undisturbed for 20 minutes.
3. Continue to cook for 15 more minutes at this point, stirring the pan every 5 minutes for a total of 3 times.
4. Stir in balsamic vinegar and cook for about 5 more minutes. This is the point where the onions have the optimum texture and taste. Let cool to room temperature before using, but it’s best to do it the day before.

Blind Bake the Pie Crust:

1. Preheat the oven to 425ºF with an oven rack set on the lowest rack and one on the middle rack.
2. Remove the pie crust from the refrigerator and let it sit for about 5 minutes, so it is easier to roll. 
3. Roll out the pie dough to an 11″ circle and line a 9” tart pan with a removable bottom. Prick the bottom of the dough all over with a fork. Place the pie in the freezer for 10 minutes.
4. Once chilled, line the pie dough with a parchment round piece of paper and then add pie weights (or dry rice or beans or lentils) to fill, making sure to push the pie weights to the edges, too.
5. Bake for 20 minutes, then remove from the oven and carefully remove the parchment and pie weights. Return to the oven and bake for an additional 5-7 minutes.

Make and Bake the Filling:

1. Place the blind-baked tart crust on a rimmed baking sheet until ready to use and lower the oven temperature to 375ºF.
2. Crack eggs into a large bowl, and whisk together with the milk, cream, salt, pepper and nutmeg.
3. Sprinkle the ¾ cup of the caramelized onions in the bottom of the baked tart shell. Then sprinkle the cheese on top of the onions.
4. Pour in the egg mixture, taking care not to overfill it. You may have some egg mixture leftover, depending on if your crust shrank at all during the blind bake. Optionally, you can add some caramelized onions on top of the egg mixture for visual appeal.
5. Bake the quiche in the middle oven for 35-40 minutes, or until the top has fully puffed up and is lightly golden all over.
6. Let the quiche sit for 5-10 minutes before slicing it.
7. Serve the quiche warm or at room temperature.