Add butter, onions, spices and crushed tomatoes to slow cooker. Cook on high for 2 hours, stirring occasionally.
Blend the sauce using an immersion blender (this is an optional step to make the sauce smooth and eliminate bits of onion). Add chicken to sauce, then cook on high for 1 more hour.
While chicken is cooking, add basmati rice to a pot on the stove or in a large rice cooker and cook according to package directions (usually 15-20 minutes until water is absorbed).
When chicken is done cooking, stir in heavy cream. Serve butter chicken with rice and naan, garnishing with cilantro. Enjoy!
Video
Notes
Nutritional info is for 1/6 of butter chicken mixture and 1/2 cup rice.