Place the chicken pieces, chopped onion, minced garlic, grated ginger, tomato sauce, butter cubes, garam masala, cumin, paprika, turmeric, salt, and black pepper into the slow cooker. Stir to combine everything evenly.
Cover and cook on low for 6 hours until the chicken is very tender and the sauce has thickened slightly.
Stir the heavy cream into the slow cooker and let it warm through for 10 more minutes on low.
While the chicken finishes, combine the basmati rice and water in a medium pot. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes until the rice is fluffy and the water is absorbed.
Warm the naan bread in a dry skillet or microwave. Serve the butter chicken over the rice with naan on the side.