Cut the croissants into 1-inch cubes, and spread in the bottom of the pan. Sprinkle the croissant cubes with cheese, ham and onion.
In a large bowl, whisk together the eggs, milk, sour cream, Dijon, salt and pepper until well-blended. Pour the mixture on top of the ingredients in the pan. Cover with foil, and refrigerate overnight.
The next morning, set the dish on the counter for 30 minutes. Preheat the oven to 350 degrees F. Uncover the dish, and bake for 45 minutes. Then sprinkle the cheddar cheese on top and bake for 5 more minutes or so… just until the cheese is melted and the casserole tests done with a knife inserted into the center coming out clean.