To make crust, combine the dry ingredients into a bowl and cut in butter with your fingers (or a pastry blender.) Once butter is broken down to the size of peas, add water to the mix and stir/knead until the dough comes together. Roll out onto a flour surfaced and gather into a disc. Wrap with plastic wrap and place in refrigerator for 20 minutes to chill. Pre-heat oven to 425˚. Roll out dough and crimp into an 11" quiche pan. If you have dough left over, store in the freezer for later use. Parbake the crust for 10-15 minutes, just until the crust gets a little brown color. Remove from oven and set aside on a baking tray.
To caramelize onions, slice onions into an even 1/4" slice and heat a medium pan with 2 tablespoons olive oil over medium low heat. Start cooking onions, stirring occasionally. After onions have cooked for 15-20 minutes, add salt. Continue to let cook, letting more time go by before you stir the onions (I go for about 15-20 minute increments.) Continue to cook and stir onions until golden, soft, and cooked down- usually 60-75 minutes. Set aside.
To prepare sweet potatoes, peel and cut into 1/4" slices. Toss with one tablespoon of olive oil and place on a covered baking tray. Roast for 15-20 minutes or until sweet potatoes are soft and begin to brown. The crust, onions, and sweet potatoes can be done all at the same time.
Reduce or preheat oven to 375˚.
Once these three elements are ready, layer the sweet potatoes in the bottom of the crust. Sprinkle caramelized onions and gorgonzola cheese on top. Finally, whisk eggs, milk, salt, pepper, and nutmeg together. Pour over filling ingredients. It is at this point you can cover the quiche, place in the refrigerator, and let sit for up to day before baking.
To bake, carefully transfer quiche to the oven. Bake for 45-55 minutes until the filling domes up and barely jiggles. Remove from the oven and let sit for 10-15 minutes before serving.