Place chicken breasts in slow cooker and sprinkle cumin, garlic salt, chili powder, and black pepper over them. Add diced tomatoes on top and cook on low for 6-8 hours or high for 4-6 hours.

Boil a large pot of water. Trim tops off bell peppers and remove seeds. Boil peppers for 5 minutes until slightly tender, then set aside to cool.

Once chicken is cooked, shred it in the slow cooker. Mix with black beans, pre-cooked rice, and half of the cheese in a bowl.

Preheat oven to 350°F (175°C). Fill each pepper with the chicken and rice mixture, topping with extra cheese if desired. Place tops back on.

Arrange peppers in a baking dish and bake for 30 minutes or until peppers are tender and cheese is melted.