Put the diced onion into the slow cooker. Place the chicken thighs on the onions and add the remaining ingredients to the slow cooker. Cook covered on high for 3 hours on High. Vent the slow cooker (move the cover off kilter) and cook for 30 more minutes. Cook time will vary based on the size of your chicken thighs. Cook until the internal temperature reaches 165 degrees. Garnish with chopped parsley and serve hot.