Place the bacon and onion in a large, deep skillet. Season with garlic powder and pepper. Cook over medium-high heat until evenly brown. Drain, add mushrooms and set aside. Slice each bagel into 4 thin slices.
Arrange 6 bagel slices in the bottom of a lightly greased 9x13 inch-baking dish. Cover with the bacon and onion mixture, followed by the cheddar cheese. Top with remaining bagel slices.
In a medium bowl, whisk together the eggs, milk, parsley, salt and pepper. Pour the egg mixture over the bagel layers. Cover, and refrigerate 8 hours or overnight.
Preheat oven to 400-degrees. Uncover the chilled bagel dish, and bake in the preheated oven 40 to 45 minutes, or until eggs are firm. Sprinkle with parmesan cheese, and serve warm.