Slice the mushrooms thinly and mince the onion and garlic. Place into a non-stick skillet with the olive oil over medium heat. Stir occasionally until the onions are translucent, approximately 7-10 minutes.
While the onions and mushrooms are cooking, whisk together the eggs with the milk with a few dashes of salt and pepper
In a pie dish, spray non-stick cooking spray, and spread the thawed spinach along the bottom of the pan. Layer the cooked mushrooms and onion on top, and add half of the feta cheese.
Pour the eggs into the pie pan. Add tomato slices and mozzarella.
Place into the preheated oven and cook for 40 minutes to an hour, until the top is golden brown and the eggs are set.