Spray 5- to 6-quart slow cooker with cooking spray. Place onion and garlic in slow cooker.
In small bowl, mix thyme, basil, oregano, salt and pepper. Rub each piece of chicken with herb mixture. Arrange chicken pieces over onion, in single layer if possible.
Cover; cook on High heat setting 3 hours 30 minutes (or Low heat setting 6 to 7 hours) or until juice of chicken is clear when thickest pieces are cut to bone (at least 165°F).
Set oven control to broil. Line broiler pan or cookie sheet with foil. Using slotted spoon, remove chicken from slow cooker and place, skin side up, on pan. Broil with tops 6 inches from heat 4 to 6 minutes or until golden brown.