Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, combine the croissant pieces, diced ham, cheddar cheese, and mozzarella cheese. Mix well and spread evenly in the prepared baking dish.
In another bowl, whisk together the eggs, milk, Dijon mustard, garlic powder, onion powder, salt, and pepper until well combined.
Pour the egg mixture evenly over the croissant and ham mixture in the baking dish. Gently press down to ensure the croissants are soaked in the egg mixture.
Cover the dish with aluminum foil and let it sit for 15-20 minutes to allow the croissants to absorb the liquid.
Bake covered for 25 minutes, then remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and the casserole is set.
Remove from the oven and let it cool for a few minutes. Garnish with chopped parsley if desired before serving.