Preheat oven to 350 degrees F.
Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
Heat oil in a large skillet over medium- high heat. Add sliced mushrooms and cook until tender, about 3 minutes.
Add garlic and sauté for an additional minute.
Add spinach and cook until wilted; remove from heat and spread in the bottom of prepared pan.
In a large mixing bowl, whisk eggs and milk until well combined; pour into pan.
Sprinkle feta cheese on top.
Bake for 50-55 minutes, or until completely set.