Combine Base Ingredients: Add chicken breasts and diced onion to the slow cooker. Pour in the chicken stock, salsa, taco seasoning, and cumin. Stir everything thoroughly to ensure the seasonings are well distributed throughout the liquid, creating a flavorful cooking base for the chicken.
Slow Cook the Chicken: Cover the slow cooker and set it to LOW for 4 hours. The chicken will slowly absorb the flavors while cooking to perfection. The chicken is done when it reaches an internal temperature of 165°F and is tender enough to be easily shredded.
Shred the Chicken: Once the chicken is fully cooked, remove it from the slow cooker. Using two forks, shred the chicken into bite-sized pieces by pulling the meat apart. The chicken should separate easily as it's been tenderized during the slow cooking process. Return the shredded chicken to the slow cooker, mixing it back into the flavorful cooking liquid.
Add Rice and Beans: Turn the slow cooker setting to HIGH. Add the quick-cooking rice and rinsed black beans to the mixture. Stir thoroughly to combine all ingredients. Cover and continue cooking for approximately 40 minutes, until the rice has absorbed most of the liquid and is tender.
Add Cheese Topping: Evenly distribute the shredded Colby jack cheese over the top of the chicken and rice mixture. Replace the cover and allow it to cook for a few more minutes until the cheese is completely melted and deliciously gooey.
Serve and Garnish: Portion the burrito bowl mixture into individual serving bowls. Allow each person to customize their bowl with optional toppings such as sliced black olives, green onions, and sour cream to enhance the flavor and add textural contrast.