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 90 minutes, 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 the heavy cream into the sauce, and whisk until the sauce is creamy. Cut the chicken into bite-sized pieces and return to the sauce. Serve with basmati rice and naan bread, garnish with fresh parsley, mint, or cilantro.