Heat oven to 375. Lightly spray a 9" deep dish pie pan. Place pie crust in pan.
In lg skillet, melt butter over medium heat. Saute garlic and onion until browned. Stir in spinach, mushrooms, feta cheese and 1/2 cup cheddar cheese. Season w/ salt and pepper.
Spoon mix into crust. In bowl, whisk together egg and milk. Pour into pie shell. Place pie pan onto a baking sheet. bake 15 mins. Sprinkle w/ remaining cheddar cheese. Bake 40 mins, or until set in center. Cover w/ foil if browning too quickly. Take off foil the last 10 mins.