Get ready: Make your crust if needed and be sure it is well chilled for at least 1 hour. When ready to make the quiche, preheat your oven to 400 degrees F.
Blind bake the crust: Lightly flour your work surface and a rolling pin (if you don’t have a rolling pin, a wine bottle will work in a pinch). Roll out one crust into an 11-inch circle. Roll the dough away from you, rotating the dough a quarter turn every few rolls. Fit into a 9-inch pie dish and fold or crimp the edges as desired. (If doing the oil pie crust recipe, press the dough directly into the dish as directed).
Line the crust with aluminum foil or parchment paper (if using parchment paper, crinkle it up first so that it is easier to shape). Fill with dried beans or pie weights. Bake until the edges of the crust are beginning to brown, about 15 minutes.
Carefully remove the foil and beans (the beans cannot be eaten but can be saved for future pies). Prick the pie crust all over with the tines of a fork, then return to the oven and continue baking until the bottom is lightly browned, about 6 to 8 minutes more. Set aside to cool.
Reduce the oven temperature to 350 degrees F.
Prepare the filling: Heat the oil in a large skillet or Dutch oven over medium-low. Add the onion and saute it slowly and gently.
Stir periodically, until it is completely tender caramelized, at least 30 minutes. Patience is key and will be rewarded!
Stir in the spinach a few handfuls at a time, until it wilts. Stir in the garlic, salt, pepper, and oregano. Let cook 30 additional seconds. Remove the skillet from the heat.
In a mixing bowl, whisk together the milk and eggs. Fold in the spinach mixture, goat cheese, and red pepper.
Bake the quiche: Carefully pour the filling into the crust. Bake at 350 degrees F until the center is just set, about 45 minutes to 1 hour. To check for doneness, I like to insert a sharp knife in the center. It should look moist but not runny.
Let the quiche cool for 15 minutes. Slice and serve warm or at room temperature.
TO MAKE AHEAD: Caramelize the onions up to 3 days in advance and store them in the refrigerator (or you can freeze them for up to 2 months). Prebake the crust up to 3 days in advance, and store it in the refrigerator. You can also prepare the entire quiche ahead of time. Cover it and store it in the refrigerator up to 1 day in advance, or freeze it for up to 3 months. If frozen, let thaw overnight in the refrigerator before reheating.
TO STORE: Refrigerate leftover quiche in an airtight storage container for up to 3 days.
TO REHEAT: Rewarm quiche in a pie dish in the oven at 350 degrees F or in the microwave.
TO FREEZE: Freeze leftovers in an airtight freezer-safe storage container for up to. 3 months. Let thaw overnight in the refrigerator before reheating.