Brown ground beef, onion and garlic together in large skillet. Drain the grease. Add diced tomatoes, oregano, salt and pepper to the skillet and stir to combine.
Place bell pepper halves in greased casserole dish.
Fill each pepper with ground beef mixture.
In a small bowl, stir tomato sauce, garlic powder, onion powder, salt and pepper together. Pour over the bell peppers.
Cover dish and bake at 350 degrees for 30 minutes.