Place the chicken thighs in the slow cooker and add the chopped onion, minced garlic, and grated ginger.
Sprinkle the garam masala, cumin, turmeric, chili powder, and salt over the chicken and vegetables.
Pour the tomato sauce over everything and add the butter in small pieces.
Cover and cook on low for 6 hours or on high for 3 to 4 hours, until the chicken is very tender.
While the chicken cooks, prepare the basmati rice according to package directions and set it aside.
Once the chicken is done, stir in the heavy cream and let it warm through for 10 minutes.
Serve the butter chicken over the cooked basmati rice and sprinkle with fresh cilantro.