In a medium skillet, melt butter over medium heat. Saute garlic and onion in butter until lightly browned, about 7 minutes. Stir in spinach, mushrooms, feta and 1/2 cup Cheddar cheese. Season with salt and pepper. Spoon half the mixture into each pie crust.
In a medium bowl, whisk together eggs and milk. Season with salt and pepper. Pour half into each pie crust, allowing egg mixture to thoroughly combine with spinach mixture.
At this point you can refrigerate them until you're ready to bake. I like to do this the night before so we can have an easy quiche breakfast.
Bake in preheated oven for 15 minutes. Sprinkle top with remaining Cheddar cheese, and bake an additional 35 to 40 minutes, until set in center. Allow to stand 10 minutes before serving.