Place the bacon in a large skillet. Cook over medium-high heat until evenly brown. Drain and set aside. Add the mushrooms and onion to the skillet; cook and stir until the onion has softened and turned translucent and the mushrooms are tender, about 5 minutes.
Grease a 9x13-inch casserole dish with the tablespoon of butter. Place the potatoes in the bottom of prepared dish. Sprinkle with salt, garlic salt, and pepper. Top with crumbled bacon, then add the onions and mushrooms.
In a mixing bowl, beat the eggs with the milk and parsley. Pour the beaten eggs over the casserole and top with grated cheese. Cover and refrigerate overnight.
Preheat oven to 400 degrees F and bake in the preheated oven for 1 hour (or until set).
You can mix all of your ingredients the night before and refrigerate overnight, place your casserole in the oven in the morning to bake.