Place the chicken pieces, chopped onion, minced garlic, grated ginger, tikka masala paste, crushed tomatoes, salt, and black pepper into the slow cooker. Stir everything together so the chicken is coated.
Cover the slow cooker and cook on low for 6 hours or on high for 3 hours until the chicken is very tender.
Stir in the heavy cream during the final 30 minutes of cooking to create the creamy sauce.
While the sauce finishes, rinse the basmati rice and cook it in a pot or rice cooker with 3 cups of water until fluffy.
Spoon the chicken tikka masala over the cooked rice and top each serving with chopped fresh cilantro.