Preheat oven to 325°F
Chop onion
Spray casserole dish with cooking spray
Arrange 2 slices of bread on bottom of casserole dish
Sprinkle half the cheese over the bread
Sprinkle garlic powder over cheese
Arrange 2 slices of bread on top of other ingredients
Sprinkle remaining cheese over bread
Beat eggs in mixing bowl with a whisk or fork
Add milk, onion, and mustard to eggs
Mix thoroughly
Pour mixture over bread & cheese in casserole dish
Refrigerate overnight
Bake uncovered for 1 hour 15 minutes in preheated oven
Serve