Preheat oven to 400º F. Place pie plate in the oven to heat it up.
In a large bowl, whisk together eggs, pepper, and gruyere cheese. Set aside.
Heat olive oil in a large pan over medium high heat. Add mushrooms and cook until tender, about 5 minutes.
Add the spinach and stir to combine. Cook until spinach is just wilted.
Remove pan from heat and let cool slightly. Stir mushroom/spinach mixture into egg mixture. Next, using oven mitts, carefully remove pie plate from oven and spray with cooking spray. Pour egg mixture into pie plate.
Bake frittata until golden brown and no longer jiggles in the center, about 30 minutes.
Allow frittata to cool 5-10 minutes before slicing and serving.
