In a small bowl, mix together garam masala, cumin, paprika, turmeric, and cayenne pepper.
Season the chicken pieces with the spice mixture, ensuring each piece is coated evenly.
Place the seasoned chicken at the bottom of the slow cooker.
Top the chicken with diced onion, minced garlic, and grated ginger, spreading them evenly over the chicken.
Pour the tomato sauce over the chicken and spices, making sure the chicken is well-covered.
Cover the slow cooker and set to cook on low for 8 hours or high for 4 hours.
Once the chicken is tender and the cooking time is complete, gently stir in the heavy cream and season with salt to taste.
Allow the mixture to heat through for an additional 10 minutes on low.
Serve the chicken tikka masala over cooked rice and garnish with chopped fresh cilantro.