Preheat the oven to 375 degrees. Grab a baking dish while it heats up.
Wash the bell peppers. Cut the tops off and pull out the seeds. Stand them upright in the dish. Trim the bottoms a little if they wobble.
Heat the olive oil in a large skillet over medium heat. Add the ground beef and break it up as it cooks.
Add the diced onion to the beef. Cook until soft. Stir in the garlic and cook for about 30 seconds.
Season with salt, pepper, and Italian seasoning if using. Spoon off excess grease if there is a lot.
Stir in the diced tomatoes and cooked rice. Mix until everything looks evenly combined and not too dry.
Spoon the filling into the peppers. Press it down lightly. Do not pack it tight.
Cover the dish loosely with foil and bake for about 25 to 30 minutes.
Remove the foil. Add cheese on top if using. Return to the oven for 10 minutes, until peppers are tender.
Let the peppers rest for a few minutes before serving. They will be hot.