Wash the bell peppers, cut the tops off, and remove the seeds and membranes. Set aside.
In a mixing bowl, combine the ground beef, cooked rice, diced tomatoes with their juice, diced onion, minced garlic, Italian seasoning, salt, and black pepper. Mix well until fully combined.
Stuff the mixture into each bell pepper until they are filled to the top. Place the stuffed peppers upright into the slow cooker.
Cover the slow cooker with its lid and cook on low for 6-8 hours, or on high for 3-4 hours, until the peppers are tender and the beef is cooked through.
If using, sprinkle shredded mozzarella cheese on top of the stuffed peppers 30 minutes before serving, allowing it to melt.
Once finished, carefully remove the peppers from the slow cooker and place them on a serving plate. Serve hot.
Optionally, substitute ground beef with ground turkey or chicken for a lighter version.
Feel free to add additional vegetables like corn or black beans to the stuffing mixture for more flavor and nutrition.
Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat in the microwave or slow cooker before serving.