Skinny Crustless Spinach Quiche

Preheat the oven to 375 degrees F (190 degrees C). Spray a 9x13-inch baking dish with cooking spray.

Heat butter in a skillet over medium heat. Saute mushrooms, onion, and garlic in the hot pan until onion is tender, about 7 minutes. Add spinach to the onion mixture and cook over low heat until spinach is limp and most of the water has evaporated, 5 to 7 minutes.

Place egg whites into a bowl and add milk; whisk together. Add 1/2 cup Cheddar cheese and feta cheese. Stir in spinach mixture until thoroughly mixed. Season with salt and pepper. Pour mixture into the prepared baking dish.

Bake in the preheated oven for 15 minutes. Remove from the oven and sprinkle remaining Cheddar cheese over quiche. Return to the oven and continue to bake until center is set, about 40 minutes.