Preheat the oven to 350. Cook 2 cups of rice according to the packaging and set aside.
Prepare your peppers by cutting off the tops and removing the seeds. Fill a large pot with water and bring it to a boil. Boil the peppers for 5 minutes. Place them upside down on a paper towel to dry.
In a large skillet, heat 1 Tablespoon olive oil. Add the onion and sauté until translucent.
Add the ground beef to the skillet. Cook until brown and cooked through. Drain any excess grease.
Add the diced tomatoes, Italian seasoning, salt, pepper, rice and ½ cup of mozzarella cheese. Stir to combine.
Place the peppers in a 9 x 13” baking dish lightly sprayed with nonstick cooking spray. Spoon the filling into the peppers, filling each one full.
Bake uncovered in the preheated oven for 25 - 30 minutes. Add the remaining mozzarella cheese to the tops of the peppers and bake for 5 minutes more.