Combine ketchup, brown sugar, onion, celery salt, garlic, and Worcestershire sauce; simmer for 15 minutes. Reserve some sauce for serving with ribs; pour remaining sauce over baked ribs. Cover and bake three more hours at 375º. Turn ribs over the last hour of baking.
Grilling method: grill ribs over indirect heat for three hours, turning every hour and basting with sauce.