Place onion in the bottom of a 5-quart slow cooker. Coat flank steak with Monteal seasoning; place on top of onion. Sprinkle soup mix over roast and add undrained tomatoes. Cover and cook on low heat setting for 8 hours (overnight).
In the morning, stir together the celery and celery soup. Spoon into slow cooker. Cover and continue cooking for 8 hours more. Serve roast hot with gravy.