Preheat oven to 350 degrees F. Heat oil over medium-high heat and add mushrooms. Cook, stirring often, 3-5 minutes. Add salt, pepper and garlic, stirring until garlic is fragrant. Add spinach, a handful at a time, and stir until wilted.
Turn up the heat, allow liquid to evaporate and remove from heat.
Beat eggs in a large bowl. Stir in milk, scallions, mushroom/spinach mixture and parmesan. Fold in mozzarella.
Coat a pie dish with cooking spray, pour in egg mixture and distribute contents evenly throughout. Cook for 25 - 30 minutes until a knife inserted in center comes out clean. If top begins to brown too quickly, cover with tin foil for last portion of cooking. Top with additional parmesan to serve.