Note about temperatures of ingredients. The pie crust will tend to slip down the sides of the pan, causing your filling to run over, especially if you make an all butter crust as I usually do. If the crust is cold when it goes in the oven, it will have a better chance of setting up before the butter melts, preventing slippage.
If the milk and eggs in the filling are cold when it goes into the oven, it will take a lot longer to bake, causing the crust to over-brown and the filling to set unevenly. It will seem to take forever for the center to set, long after it is set around the edges.
Notes from the Test Kitchen:
This quiche recipe is perfect for your next brunch. I added the optional feta cheese and bacon and it was delicious. I highly recommend adding them. Serve with a green salad and you'll have an impressive addition to your brunch menu.
Preheat oven to 350 degrees. If you have a baking stone, put it on the rack to preheat.
Line a 9-inch deep dish pie pan with crust, flute edges high to hold possible overflow during baking. Refrigerate crust 30 minutes or more. Crust should be cold when the quiche goes into the oven to prevent its sliding down.
Heat a good sized skillet. When pan is hot, add butter. When butter is melted, add sliced onions. Sauté until they start to brown. Add mushrooms and continue to sauté and stir until all is well browned. Remove from heat.
Cook chopped spinach in a saucepan in water until heated through (water boils) stirring to break up any frozen parts. Drain water from spinach in a sieve and press spinach to dry well. Add spinach to pan with mushrooms and onion. Stir to mix in uniformly.
Add Swiss cheese to pan with vegetables and stir to a uniform distribution of ingredients.
Put vegetable and cheese mixture into pie crust and spread out evenly.
Add milk to well beaten eggs and whisk a bit to fully incorporate. Add salt and pepper to taste. Pour milk and egg mixture over vegetables in pie pan. It should fill to top, but don’t go over. You might want to set quiche in a sheet pan/jelly roll pan to catch any overflow.
Place on preheated baking stone in the oven. Bake about 50-60 minutes, checking frequently, until crust is nicely browned and center of quiche is set. Cool on rack.