Skinny Crustless Spinach Quiche Recipe

Ingredients

• cooking spray
• 3 tablespoons unsalted butter
• 6 1/2 ounces mushrooms, chopped
• 1/2 cup chopped onion
• 3 tablespoons minced garlic
• 1 (10 ounce) package fresh spinach, chopped
• 1 cup egg whites
• 1 cup skim milk
• 1 (8 ounce) package shredded fat-free Cheddar cheese
• 6 ounces crumbled fat-free feta cheese
• salt and ground black pepper to taste