Cut the top off of each bell pepper and scoop the seeds and white membranes out. Cut up some of the peppers that is around the tops to equal about 1/2 cup diced pepper.
Brown the ground beef in a large nonstick pan, crumbling it as it cooks.When it is about halfway cooked, add the onion, diced bell pepper and garlic.Continue to cook until there is no more pink in the ground beef.
Add 1 cup marinara sauce, the salt, pepper, Italian seasoning, and white rice. Stir well. Spoon mixture into the peppers and place them in a 6-quart slow cooker.
Pour the remaining marinara sauce around the peppers and spoon any extra filling around the peppers as well.
Cover and cook on LOW for 5 hours.
Remove the top and sprinkle on the mozzarella cheese. Cover for 5 minutes to melt the cheese.
Notes
You can skip pre-cooking the ground beef and add 1 hour to the cook time.Make it spicy. Add half a teaspoon of crushed red pepper flakes to the filling.