Prepare Chicken: In a slow cooker, place chicken breasts and sprinkle with cumin, garlic salt, chili powder, and black pepper. Pour canned diced tomatoes with green chilies over the chicken. Cover and cook on low for 6-8 hours or high for 4-6 hours, until the chicken is tender. Once cooked, shred the chicken with forks and let it rest in the juices for extra flavor.

Prep Peppers: Boil a large pot of water over medium heat. Meanwhile, rinse the bell peppers, slice off the tops, and carefully hollow them out. Once the water reaches a rolling boil, gently add the peppers and their tops, boiling for 5 minutes until slightly tender. Remove, drain, and set aside to cool before stuffing.

Make Filling: In a large mixing bowl, combine the cooked Mexican rice, shredded chicken, black beans, and shredded cheddar cheese. Use a fork to mix everything until well-blended. Taste it for seasoning.

Stuff Peppers: Carefully spoon the chicken and rice filling into each hollowed bell pepper, packing it tightly. If desired, sprinkle a little extra cheese on top. Replace the bell pepper tops.

Cook Peppers: Preheat your oven to 350°F (175°C). Arrange the stuffed peppers upright in a baking dish. Bake for 30 minutes, or until the cheese is melted and bubbly, and the peppers are cooked to your desired tenderness.

Serve and Enjoy: Once cooked, let the peppers cool for a few minutes before serving. These festive stuffed peppers are perfect for sharing!