Cut croissants into 1 inch cubes (about 4 cups, loosely packed) and spread out on the bottom of the prepared pan. Sprinkle evenly with cheese and ham.
Combine eggs, milk, sour cream, Dijon mustard, salt, and pepper in a bowl. Beat the mixture until well blended using a wire whisk or an egg beater.
Pour egg mixture evenly over the top of the croissants, cheese, and ham.
Cover tightly with aluminum foil and place in the refrigerator for 8 hours.
When ready to bake, set the dish out of the refrigerator on the counter, Preheat the oven to 350 degrees F (175 degrees C).
Uncover casserole, place in the center of the preheated oven, and bake until the casserole is set, and a knife inserted into the middle of the casserole comes out clean, 45 to 55 minutes.
Cut into serving pieces and garnish with chopped green onions.