Grease one 6 quart casserole dish with butter. Place bacon in a large, deep skillet. Cook over medium high heat until evenly brown. Drain and set aside. In a separate skillet, melt the butter and fry onions and mushrooms until tender. Place potatoes in bottom of prepared dish. Sprinkle with salt, garlic salt, and pepper. Top with bacon then onions and mushrooms. In a large bowl, beat eggs with milk and parsley. Pour over casserole. Top with grated cheese. Cover and refrigerate overnight. Bake at 400 degrees F (200 degrees C) for 1 hour or until set. If preparing and baking the same day, cook for 45 minutes.