Place the chicken thighs, onion, garlic, ginger, crushed tomatoes, butter, garam masala, cumin, smoked paprika, and salt into the slow cooker. Stir to combine everything evenly around the chicken.
Cover and cook on low for 6 hours until the chicken is very tender.
Remove the chicken to a cutting board, cut it into bite-sized pieces, and return it to the slow cooker.
Stir the heavy cream into the sauce until fully blended and the sauce turns a lighter orange color.
Spoon the butter chicken over prepared cauliflower rice and sprinkle with fresh cilantro before serving.