Grease a 9x13-inch baking dish with non-stick spray.
Lay the bread slices in a single layer in the dish.
In a large bowl, whisk together eggs, milk, mustard, salt, pepper, and onion powder.
Sprinkle the diced ham and bell peppers over the bread.
Pour the egg mixture evenly over the top.
Top with shredded cheese and drizzle with melted butter.
Cover with foil and refrigerate overnight.
Preheat oven to 350°F (175°C).
Bake covered for 45 minutes, then uncover and bake for another 15 minutes or until golden and set.
Garnish with parsley if desired and serve warm.