Add the chicken pieces, chopped onion, and minced garlic to the slow cooker.
Pour in the tomato sauce, then add the heavy cream, butter, garam masala, cumin, and salt.
Stir everything together until the chicken is coated evenly with the sauce mixture.
Cover and cook on low for 6 hours, until the chicken is fully cooked and tender.
Serve the butter chicken over the cooked rice with the warmed naan on the side.