Add onion and ground beef to large skillet or frying pan on medium high heat. Cook until meat is browned, breaking up with meat chopper (or wooden spoon), about 5-7 minutes.
Add diced tomatoes, sauce, and peppers. Once it comes to a boil, reduce heat to simmer (about medium low). Cook for 10-15 minutes, stirring occasionally, until tomatoes and peppers are cooked and sauce is thickened. If sauce becomes too thick, add ¼ cup water and cook to desired thickness.
Meanwhile, butter open sides of buns and place on a baking tray. Sprinkle lightly with garlic powder and parsley if using. Bake for 6-8 minutes, or until lightly browned.
Serve meat sauce over buns, open faced or like a burger.
Notes
*Use any combination of two yellow, orange, red, or green bell peppers