IF YOU GIVE A BLONDE A KITCHEN
In a skillet, melt butter over medium heat. Cook onion and mushrooms until tender, about 5 minutes.
Next, add spinach and cook until wilted. Remove from heat.
In a large bowl, vigorously whisk eggs and egg yolk.
Add salt, pepper, cayenne, and whisk until combined.
Stir in the cheese and cooked vegetables. Mix until combined–this is going to be the base for the quiche.
Transfer filling to partially baked crust. Bake quiche at 375° F for 40 minutes.
Bake until the center barely jiggles when gently shaken. Allow the quiche to sit for 10 minutes before cutting and serving.