Thaw spinach and squeeze excess moisture/water from spinach.
Preheat the oven to 350ºF.
Slice mushrooms thinly, mince garlic and chop onion.
Add olive oil to a non-stick skillet and sauté onion and garlic for about 2 minutes before adding in the sliced mushrooms. Sautee for about 5-10 more minutes until mushrooms have browned.
Coat a pie dish with non-stick spray or olive oil. Place the squeeze-dried spinach in the bottom of the pie dish. Place the sautéed mushrooms on top of the spinach, followed by a layer of the sliced emmental (swiss) cheese. You can substitute this cheese for any other sliced cheese.
In a medium bowl, whisk together the eggs, milk, and and remaining cheeses. Leave some of the cheese out to sprinkle on top later. Season lightly with pepper and salt. Pour the egg mixture into the pie dish. Top with the the left over cheese.
Bake the crustless quiche for 40-50 minutes, or until the top is golden brown