Cut The Brisket In Half; Rub With Liquid Smoke, Salt And Pepper. Place In A 3-Qt. Slow Cooker. Top With Onion. Combine The Ketchup, Mustard And Celery Seed; Spread Over Meat. Cover And Cook On Low For 8-9 Hours. Remove Brisket And Keep Warm. Transfer Cooking Juices To A Blender; Cover And Process Until Smooth. Serve With Brisket.