Pound each steak until thinned to about 1/4-inch. Saute bacon,( celery, and green onion if using); drain off fat. Combine sauteed bacon and vegetables with DRY stuffing mix (including seasoning with the mix, if any).
Stir in half the cream of celery soup and enough water to moisten.
Place sliced onions in the bottom of the slow cooker.
Spread stuffing mixture on each flattened steak; roll up and secure with toothpicks; place on top of onions.
Spoon remaining cream of celery soup over the rolls, then pour the tomatoes over all.
Cover and cook 8 to 10 hours on low, or 4 to 6 hours on high.