Preheat oven to 350 degrees then spray an 11x17" baking dish (or 2 medium-size baking dishes) with nonstick spray and set aside.
Heat 1 Tablespoon oil in a very large skillet over heat that's just a touch above medium. Add mushrooms then saute until they have released their liquid and the liquid has cooked off. Season with salt and pepper then continue to saute until the mushrooms are golden brown and tender. Scoop into a bowl then set aside.
Heat remaining Tablespoon oil in skillet then add peppers and onions. Season with salt and pepper then saute until crisp tender, adding an extra drizzle of extra virgin olive oil and/or turning the heat down slightly if the vegetables begin to burn. Spread vegetables onto a plate to cool slightly.
In a very large mixing bowl, whisk together eggs, milk, salt, and pepper (I usually use ~3/4 teaspoon salt). Add hash browns and 8oz of each cheese to the bowl then stir to combine. Add sauted vegetables then stir to combine.
Pour mixture into prepared baking dish then sprinkle the top with remaining shredded cheese and cover with a piece of nonstick sprayed foil. Bake for 50-60 minutes, or until the center of the casserole is set, then remove foil and bake for an additional 2-3 minutes or until cheese is fully melted. If you are using two smaller baking dishes, start checking for doneness around 40 minutes. Let casserole sit for 10 minutes before slicing and serving.
Notes
I like to serve each slice of this breakfast casserole with chopped green onions, guacamole, or salsa.