Preheat the oven to 400 degrees.
Heat the butter in a large skillet over medium-high heat. Add the mushrooms and cook, stirring occasionally, until golden brown, about 7 minutes.
Add the chopped baby spinach, minced garlic and crushed red pepper flakes and cook, stirring constantly, for 1 minute. Season with sea salt and freshly cracked pepper, to taste.
Coat a pie pan with cooking spray then carefully unroll the pre-made pie crust and place it in the pie pan. Crimp the edges.
Combine the eggs, milk, sea salt, and freshly cracked pepper, to taste, together in a bowl and whisk until very well combined.
Sprinkle most of the mushrooms, spinach and some of the shredded fontina cheese in the bottom of the pie.
Add the well whisked egg mixture.
Sprinkle with the remaining mushrooms, spinach and cheese on top.
Carefully place into the oven and bake for 15 minutes then reduce the heat to 375 and continue to bake for 25-30 minutes, or until the top is set and not wiggly and the eggs are cooked through. Don’t overcook.
Let the quiche cool for at least 15 minutes before slicing and serving. Enjoy.