Place the boneless, skinless chicken breasts into your slow cooker. Sprinkle cumin, garlic salt, chili powder, and black pepper over the chicken, then add the diced tomatoes with green chilies. Set your slow cooker to low and allow to cook for 6-8 hours (or high for 4-6 hours) until the chicken is tender enough to shred easily.

While your chicken is cooking, bring a large pot of water to a boil. Carefully slice off the tops of the bell peppers, removing the seeds and membranes. Boil the hollowed peppers and their tops for about 5 minutes until they become tender, then remove and let them cool.

In a mixing bowl, combine 2 cups of your cooked Mexican rice with the shredded chicken from the slow cooker, which should be warm and flavorful. Add the rinsed black beans and shredded cheddar cheese, then stir everything together until well mixed.

Take each cooked bell pepper and fill them generously with the chicken and rice mixture. Pack the filling down gently. Add a little extra shredded cheddar cheese on top of each stuffed pepper, then place the tops back on.

Preheat your oven to 350°F (175°C). Arrange the stuffed peppers in a baking dish, cover with foil, and bake for 30 minutes until the cheese is melted and the peppers are tender.