Whisk together eggs, milk, mustard, smoked paprika, salt, and pepper. Add egg mixture and 1 cup of the cheese to bowl with onion mixture, and stir to combine. Arrange croissants, slightly overlapping, in 2 rows in a lightly greased 11- x 7-inch baking dish. Pour mixture over croissants. Sprinkle with remaining 1⁄2 cup cheese. Cover with aluminum foil, and chill 8 hours or up to overnight.