Preheat oven to 350 degrees F.
Unroll pie dough into pie shell, crimping the edges with your hand or a fork. If pie shell is pre-formed, unwrap and set aside.
Sauté the onion and garlic in olive oil in a skillet on the stovetop until golden.
Add spinach and mushrooms. Cook until spinach is wilted and mushrooms are tender but not mushy.
In a bowl, combine eggs, milk, and seasoning. Mix well.
In another bowl, toss cheese with flour.
Place sautéed onions, garlic, and spinach on the bottom of the pie crust, and spread around to cover the crust.
Spread cheese mixture on top of the spinach and mushrooms.
Top cheese mixture with the egg mixture.
Gently stir the spinach, mushrooms, cheese, and egg mixture in the pie crust to distribute throughout the quiche.
Bake quiche at 350 degrees F for approximately 50-60 minutes until top is lightly golden. Insert a toothpick or knife into the quiche's center and ensure it comes out clean.
Let quiche stand for 10 minutes to cool before serving. You can also serve cold. Another great tip – make this recipe a day before and keep it in the fridge. Let the flavors marinate overnight. Then, heat before serving.