Heat oil in a medium sized pan. Add onions and peppers and sauté for 2-3 minutes until onions start to turn translucent.
Add mushrooms and garlic and cook until mushrooms are cooked through and all water in pan has evaporated, approximately 5 minutes. Remove pan from heat and add spinach. Stir for 1 minute longer until spinach is wilted and set aside.
Crack eggs and whisk until frothy. Add milk, salt, pepper, and nutmeg and whisk until frothy (2-3 minutes).
If veggies are watery, remove any excess liquid from the cooked veggies by squeezing them in a kitchen towel or paper towels over the sink (this keeps the crust from getting soggy).
Remove frozen pie crust from freezer or unroll refrigerated pie crust and press into 9-9 1/2" inch pie pan.
Place veggies on top of crust in pie pan in an even layer. Sprinkle cheese on top. Pour egg mixture evenly over the veggies and cheese.
Bake for 35-40 minutes until starting to brown on top. Remove from oven and let sit for 5-10 minutes before cutting and serving.
*Recipe has been adjusted to use less liquid to fit into a 9" pie pan.
Place quiche pan on a baking sheet to make it easier to take in and out of the oven.
If using homemade crust and don't want to par-bake, place pan in the bottom third of oven to help the bottom crust become crisper.
Cook vegetables down all of the way to avoid a watery quiche.
To make ahead all filling steps can be done ahead and the vegetables, cheese and egg / milk mixture can be refrigerated together in a sealed container for up to 48 hours. When ready to use, just pour into a pie crust and bake.
Freeze the crust and filling separately. Prep all of the filling (veggies, milk/egg, cheese) and put all into a bag or freezer safe container. Lay flat and freeze for up to 3 months. To use, let the filling defrost in the fridge overnight or place the container of filling into a pan of water to defrost faster. Pour the filling into the pie crust and bake as you would normally.
Freezing leftovers can be frozen as individual servings in tightly sealing containers for 1-2 months. Defrost in the refrigerator overnight or on the defrost setting on your microwave.
Reheating leftovers can be done by microwaving each serving for 30-60 seconds. It helps to cut the quiche slice into several pieces so it microwaves more evenly.
To reheat a full quiche that you've already fully baked, let it warm in a pre-heated 350° (175° C) oven until the center is warm. You can check by inserting an instant read thermometer into the center and it will read 165° F (73° C). For a cold quiche, this will take 30-35 minutes. It will heat much faster if the quiche is already room temperature. If edges are getting too dark, foil strips can be crinkled around them to prevent burning.