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Preheat the oven to 400 degrees.
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Heat the butter in a large skillet over medium-high heat. Add the mushrooms and cook, stirring occasionally, until golden brown, about 7 minutes.
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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.
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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.
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Combine the eggs, milk, sea salt, and freshly cracked pepper, to taste, together in a bowl and whisk until very well combined.
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Sprinkle most of the mushrooms, spinach, and some of the shredded fontina cheese in the bottom of the pie.
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Add the well-whisked egg mixture.
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Sprinkle with the remaining mushrooms, spinach, and cheese on top.
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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.
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Let the quiche cool for at least 15 minutes before slicing and serving. Enjoy.