In a medium bowl, mix the garlic, ginger, apple juice, brown sugar, ketchup, cider vinegar and soy sauce together. Place the chicken thighs into the crock pot. Pour the mixed sauce into the crock pot and over the chicken, then use tongs to coat the chicken thighs in the sauce. Cook on high for 4 hours, until the chicken is tender. Sprinkle with red pepper flakes and chopped green onion on top. Serve with steamed rice.