Begin by preparing the bell peppers. Cut the tops off and remove the seeds and membranes. Set aside.
In a large bowl, combine the ground beef or turkey, cooked rice, diced onion, minced garlic, drained diced tomatoes, 1/2 cup mozzarella, Parmesan cheese, tomato paste, Italian seasoning, paprika, salt, and pepper. Mix until well combined.
Stuff each bell pepper with the meat and rice mixture, pressing down gently to pack it in.
Pour half of the marinara sauce into the bottom of the slow cooker. Arrange the stuffed peppers upright in the sauce.
Top the stuffed peppers with the remaining marinara sauce, ensuring they are well coated.
Cover the slow cooker and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the peppers are tender and the filling is cooked through.
About 15 minutes before serving, sprinkle the remaining 1/2 cup of mozzarella on top of the peppers. Cover until the cheese is melted and bubbly.
Once cooked, carefully remove the peppers from the slow cooker and let them sit for a few minutes before serving. Enjoy!