Grease a 9x13 inch baking dish
Whisk eggs, milk, salt, mustard, and garlic powder in a large bowl
Stir in cheeses, ham, and scallions
Tear bread into 1-2 inch chunks and place in baking dish
Pour egg mixture over bread, fold gently to distribute ingredients
Cover and refrigerate (minimum 1 hour, up to 18 hours)
Preheat oven to 350°F and bake for 45-50 minutes until puffy and golden