In a medium skillet, sauté the onions and garlic in the coconut oil until softened, then stir in the ginger, tomato paste, and garam masala until fragrant. Turn heat off.
Cut the chicken into 1 inch size pieces and transfer them to the slow cooker. Add the onion mixture, stir in the diced tomatoes with their juices and stir until well combined.
Cover the slow cooker and cook for 4 hours on high or 8 hours on low. Fifteen minutes before the end of cooking, stir in the coconut milk. If you prefer a thicker sauce, leave the slow cooker uncovered and turn to high for the last 15 minutes. Taste and add more salt if needed.
Serve over cauliflower rice or brown rice with fresh cilantro sprinkled on top.