1. Place onion a 4-quart oval electric slow cooker. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add half of chicken, and cook 3 to 4 minutes on each side or until golden brown. Place chicken in a single layer on top of onion. Repeat procedure with remaining chicken.
2. Combine ketchup and next 7 ingredients (through hot sauce); pour over chicken. Cover and cook on LOW for 4 hours until chicken is tender and sauce is thick.
3. Remove chicken from slow cooker. Shred chicken with 2 forks, and stir into sauce. Spoon 3/4 cup chicken mixture onto bottom of each bun; cover with bun tops.