Mix tomato paste, coconut milk, garlic, ginger, cumin, turmeric, garam marsala, and salt in slow cooker. Add cinnamon stick, bay leaves, chicken, and onions. Cover and cook 8hrs on low or 4hrs on high. When chicken is cooked through and tender, remove cinnamon stick and bay leaves. Shred chicken. Serve with fresh cilantro over rice or cauliflower rice.