Preheat oven to 425 degrees F.

Use a sharp knife to slice the tops off the peppers, removing any stems, seeds, or membranes from inside each pepper. Dice any extra bell pepper flesh from the tops of the peppers to add to the filling later.

Arrange the peppers in a casserole dish with the cut side up. Add about ½ inch of water to the dish, then cover with aluminum foil and bake the peppers for 20 minutes to partially bake them.

While the peppers bake in the oven, cook the ground beef, onion, garlic, and reserved diced bell peppers in a large skillet, stirring and breaking up frequently, until the meat is browned. Add zucchini and cook for another 1-2 minutes.

Remove the meat from the heat and stir in the rice, tomatoes, and salt. Taste and adjust seasoning as needed.

Remove the partially baked peppers from the oven and discard the water. Fill with the rice and beef filling, then top with cheese. Return to the oven and cook, uncovered, until cheese is melted and the stuffed peppers are heated all the way through, about 5 minutes.