Spray the inside of the crockpot with non-stick cooking spray.
In a large mixing bowl, combine the uncooked rice, garlic powder, onion powder, dried thyme, salt, and black pepper. Mix well.
Add the chicken broth and water to the rice mixture and stir until combined.
Add the sour cream, condensed cream of mushroom soup, and condensed cream of chicken soup to the rice mixture. Stir until smooth and well combined.
Add the chopped onion and minced garlic to the mixture and stir to combine.
Pour half of the rice mixture into the bottom of the crockpot and spread evenly.
Place the chicken breasts on top of the rice layer in the crockpot.
Pour the remaining rice mixture over the chicken breasts, spreading evenly to cover the chicken.
Cover the crockpot with the lid and cook on low for 4 to 4 1/2 hours, or until the rice is tender and the chicken is cooked through (internal temperature of 165°F).
About 15 minutes before serving, sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Cover and let the cheese melt for the remaining cooking time.
Once done, carefully remove the chicken breasts and shred them with two forks, then stir the shredded chicken back into the rice mixture in the crockpot.
Serve the casserole hot.