Preheat oven to 350 degrees. Butter four au gratin dishes. Place the bottom half of each croissant in a dish, cut side up.
Sprinkle Swiss cheese on each, saving about 1/2 cup to sprinkle on top. Follow with all of the Parmesan and Mozzarella, dividing each cheese equally among the croissants.
In a medium bowl, beat the eggs, milk, and seasonings until they are frothy. Pour 2/3 cup of the egg mixture over each croissant and replace the tops. Sprinkle the remaining Swiss cheese evenly over the tops.
Bake 20 minutes or until the eggs are set and the croissants are a golden brown.