Spray 5- to 6-quart slow cooker with cooking spray. Place onion in slow cooker. Arrange chicken pieces over onion, in single layer if possible.
Reserve 1/4 cup of the cooking sauce in small bowl; refrigerate. In separate bowl, stir together remaining cooking sauce and the thyme with whisk. Pour over chicken.
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).
Remove chicken from slow cooker to plate; cover to keep warm. Add flour to reserved cooking sauce; stir with whisk until blended. Stir into sauce in slow cooker. Cover; cook on High heat setting 10 to 15 minutes or until sauce begins to thicken.
Meanwhile, set oven control to broil. Line broiler pan or cookie sheet with foil. Place chicken, skin side up, on pan. Broil with tops 6 inches from heat 4 to 6 minutes or until golden brown. Serve chicken with sauce. If desired, season to taste with salt and pepper.