Bring a large pot of water to a boil. Cut the tops off of your peppers and pull out the seeds and membranes, creating a nice cavity inside. Dip the peppers into the boiling water, letting them cook for just 2 minutes, then pull them out and drain on paper towels.
Meanwhile, brown the ground beef and onion in a skillet, then drain any fat. Add in the garlic and stir until fragrant. Next, toss in the cooked rice, half of the tomato sauce (save the other half for later), diced tomatoes, shredded zucchini, Italian seasoning and salt. Stir to combine and cook until heated through.
Spoon the ground beef mixture into each of the prepared peppers and then place in a casserole dish. Top each pepper with about a tablespoon of the tomato sauce and cover the dish with foil.
Bake for 10 minutes, remove foil and continue baking for an additional 15 minutes. Top with the shredded mozzarella cheese and bake 3 more minutes, or until cheese is melted. Enjoy!