Spray 3 1/2- to 4-quart slow cooker with cooking spray. Place chicken in cooker. Top with onion, poultry seasoning, thyme, pepper, gravy and celery.
Cover; cook on Low heat setting 8 to 10 hours.
Gently stir frozen vegetables into chicken mixture. Increase heat setting to High. Cover; cook 30 minutes longer.
Meanwhile, heat oven to 375°F. Bake biscuits as directed on bag.
Spoon about 1 cup chicken mixture onto each of 6 plates. Top with split biscuits.