Place the bacon and onion in a large, deep skillet. Cook over medium-high heat until evenly brown. Drain, 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, and pepper. Pour the egg mixture over the bagel layers. Cover, and refrigerate 8 hours or overnight.

Preheat oven to 400 degrees F (200 degrees C). Uncover the chilled bagel dish, and bake in the preheated oven 25 to 30 minutes, or until eggs are firm. Sprinkle with Parmesan cheese, and serve warm.