Preheat the oven to 425°.
Add sausage and onions to a skillet and cook over medium high heat, crumbling as it cooks, until completely browned with crispy bits. Strain off as much excess grease as possible, then stir in the garlic and remove the skillet from heat. Set aside.
1 lb ground sausage, 1/3 cup diced yellow onion, 2 tsp minced garlic
Add both the eggs and milk to a large mixing bowl, whisking together until evenly combined.
3-4 large eggs, 1 1/4 cups milk
Add the cheese, sausage mixture, spinach, salt & pepper to the egg mixture stirring again until evenly combined.
8 oz frozen spinach, 1 1/2 cups shredded mozzarella cheese, salt & pepper
Place the pie crust on a baking sheet, then pour the egg mixture in just until filled. Do not overfill.
1 9" frozen deep dish pie crust
Carefully transfer the baking sheet with quiche to the oven so it doesn't spill. Then reduce the heat to 325°. Bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean.
Let the quiche rest for 10 full minutes after baking, allowing the filling to fully set up.
Slice and serve warm.