In your crockpot, whisk together the tomato sauce, garlic, ginger paste, cumin, garam masala, turmeric, curry powder, salt, and tomato paste.
Place the chicken and chopped onions into the sauce. Flip the chicken around so it's coated well.
Slice the butter into thin pieces and place them on top of the chicken.
Cook on low for 4–6 hours or high for 3–5 hours, until the chicken is tender.
Remove the chicken and chop it into large chunks. Stir the heavy whipping cream into the sauce, then add the chicken back in.
Spoon the butter chicken over warm rice or with naan bread.