Combine the onion, garlic, all spices, and tomato sauce in your slow cooker. Mix to combine, then add the cut up chicken. Stir, and cook on low for 6 hours (or high for 4).
Twenty minutes before you want to eat, put the yogurt into a separate bowl. Stirring continuously, add ¼ cup at a time of the hot tomato sauce to the yogurt, until you've added about 2 cups. Then, pour the entire thing back into the crock pot and stir to combine.
Cook another 15-20 minutes, or until heated through. Add salt and pepper to taste.
Serve garnished with lime and cilantro, along with naan and rice.
Recipe by The Kitchen Paper at http://c02.ddd.mwp.accessdomain.com/healthy-slow-cooker-indian-butter-chicken/