– ½ cup butter – 1 small onion, chopped – 3 cloves garlic, chopped – 1 (10 ounce) package frozen chopped spinach, thawed and drained – 1 (4.5 ounce) can mushrooms, drained – 1 (6 ounce) package herb and garlic feta, crumbled – 1 (8 ounce) package shredded Cheddar cheese, divided – salt and pepper to taste – 1 deep-dish pie shell – 4 eggs – 1 cup milk