In a large skillet on medium heat, cook the lean ground beef.
Halfway through the browning process, as you’re breaking up the ground beef, add the diced onion. Because you are using lean ground beef you should have little to no oil created in the pan therefore you don't need to drain the fat from the meat.
While the meat cooks, cut the stems and membranes off the bell pepper allowing for a hole for the meat mixture to be stuffed into. Rinse, pat dry and set to the side.
Once the ground beef is cooked add the tomato sauce, diced tomatoes, Italian seasoning, salt and pepper.
Mix well, then add the cooked rice. Stir until rice and beef are incorporated.
Carefully begin spooning the rice/beef mixture into each of the bell peppers until the pepper is packed full.
Place the peppers in the slow cooker.
Once all six peppers are filled, carefully add 1 cup of water to the slow cooker. Place the lid on and cook on high for 4 hours.
During the last 5 minutes of cooking, remove the lid and add shredded mozzarella cheese on top of each pepper.
Place the lid back on and cook until cheese has melted (about 5 minutes).