Heat olive oil in a medium skillet. Add onions and red bell pepper, and sauté until onions are translucent, about 7 minutes. Remove from heat and let cool.
In a large mixing bowl, whisk together milk, eggs, salt, pepper and garlic powder. Stir in bread, ham, onion-pepper mix and cheese.
Pour into a greased 9x13” baking dish and cover with foil. Refrigerate at least one hour and up to overnight.
Bake at 350ºF for 45 minutes. Remove foil and bake 10-15 minutes more until golden brown. Serve immediately