To the slow cooker, add ginger, garlic, tomato sauce, spices, salt and sugar. Give everything a good stir, then add the chicken thighs and coat them in the sauce.
Scatter the butter all over the top, cover and cook on HIGH for 1.5-2 hours, or on LOW for 4-6 hours. The chicken is done when its internal temperature reaches 165°F with an instant read thermometer.
Remove the chicken, and place on a cutting board to cool enough to be handled.
Stir in the heavy cream in the sauce, and whisk until the sauce is creamy.
Cut the chicken into bite size pieces, and return to the sauce.
Serve with basmati rice and naan bread, garnish with fresh parsley, mint, or cilantro.