Take the cod-fish; cut it up into small pieces; chop the red onions; put them in a stewpan with the butter; let them brown without burning. Now add the fish and olive-oil, garlic, bay leaves, lemon, Shrewsbury tomato catsup, and saffron. Add sufficient hot soup stock to cover the whole; boil slowly for half to three-quarters of an hour; skim carefully while boiling; when ready to serve add celery tops.