Preheat oven to 350º F. Place prepared pastry shells in a mini muffin pan or on a baking sheet.
Add the sausage, onion, garlic, and mushrooms to a skillet set over medium heat. Saute until the sausage has cooked through and the vegetables become fragrant and translucent, about 6 minutes. Remove the skillet from the heat and stir in the spinach.
Whisk the eggs in a medium bowl until smooth. Add the cooked sausage mixture to the eggs. Spoon the mixture into the pastry shells.
Bake until the egg in the quiche has fully set, about 8 minutes. Remove from the oven and allow to cool slightly before serving.