In a small saucepan, add finely chopped onion and butter. Cook over medium heat until onions are tender. Add minced garlic and cook another minute. Add remaining ingredients to the saucepan, except for the chicken, and bring to a low simmer. Simmer until sauce lightly thickens. Remove from heat.
Add chicken thighs/breasts to your slow cooker and pour barbecue sauce over the chicken (about 1 cup). Cook on low for 6 hours or on the high setting for 3 hours.
When finished, using two forks, lightly shred chicken. Enjoy!