Combine garlic powder, onion powder, ground turmeric, garam masala, curry powder, ground coriander, ground cumin, paprika, ground ginger root and salt in a small bowl, whisk to combine.
Add cubed chicken to the spice bowl and mix well to coat the meat.
Add the season chicken pieces to the slow cooker. Add the chopped onion, fresh ginger, diced tomato, tomato paste and tomato sauce. Stir well.
Cover the slow cooker and cook for 4 hours on high or for 8 hours on low.
Heat the heavy cream in the microwave for 1 minute on high, and stir into the chicken tikka masala. Taste and add more salt and paprika (for spiciness) if needed.
Serve over rice garnished with fresh parsley or cilantro.
If you want to freeze the chicken tikka masala, do so before stirring in the heavy cream. The cream can be mixed after thawing, while heating the chicken tikka masala in a sauce pan over medium heat. Chicken tikka masala can be frozen to up to 3 months.