Prep all ingredients: Cut croissants into ~1 inch cubes, cube the ham and slice green onions. Grease a 9 x 13 inch rectangular casserole dish. Add cubed croissants to dish.
In a large bowl, combine and whisk eggs, milk, garlic powder, ground mustard, salt and pepper. Add egg mixture to casserole dish, making sure to pour all over as much of the croissants as possible.