To make the spice rub, combine the garlic, ginger, vegetable oil, sesame oil, soy sauce, brown sugar, and 5 spice powder in a small bowl. Stir until combined. (Grate the ginger using a small holed cheese grater, or mince with a knife.)
Rub the spice mixture all over the chicken pieces, getting between the skin and meat whenever possible.
Dice the onion and place it in the bottom of the slow cooker. Pour the rice wine over the onion, then layer in the chicken pieces. Make sure the chicken pieces are closely packed in the cooker. Cover the slow cooker and cook on high for four hours.
After four hours, the chicken will be very soft and tender. Use tongs to carefully remove the chicken from the liquid in the slow cooker and place them on a baking sheet. Adjust your oven rack so that the top of the chicken will be 5 inches from the broiler (the broiler's heat element is usually on the roof of your oven). Broil the chicken on high for 5 minutes, or until the skin is brown and crispy. Keep a close eye on the chicken while it's under the broiler because every broiler is slightly different and it can burn quickly.
**If you don't have rice wine or can't find it, you can use a dry white cooking wine or chicken broth in its place.