Place the chicken thighs, yogurt, diced onion, minced garlic, grated ginger, garam masala, cumin, paprika, turmeric, and salt into the slow cooker. Stir to coat the chicken evenly.
Pour the tomato sauce over the mixture and stir once more to combine.
Cover and cook on low for 6 hours or on high for 3 hours until the chicken is tender.
Stir in the heavy cream during the last 15 minutes of cooking.
While the sauce finishes, cook the basmati rice according to package directions in a separate pot.
Spoon the chicken tikka masala over the cooked rice and top with chopped cilantro before serving.