Place 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 potatoes in 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 (200 degrees C).
Bake in preheated oven for 1 hour or until set.
If you're baking this the same day you make it (so it's not refrigerated) reduce the bake time to 45 minutes.