In a large skillet over medium heat, sautée onions and stir until wilted and light brown, about 5 minutes. Add diced pepper and garlic and continue to saute until pepper is softened. Add the ground beef and brown, drain off liquid from pain.
Stir in the mustard, garlic powder, brown sugar and ketchup, mix thoroughly. Reduce heat, and then simmer for about 20 minutes or until ready to serve.
Butter the hamburger buns and place under oven broiler for 2 minutes, just before serving.