Warmly satisfying, this combination suggests more complexity than you might imagine—hints of mushrooms, artichoke hearts, potatoes—thanks to the soft, mysterious salsify.
Remove gills if any, from fish head; chop skeleton into manageable pieces; combine with water and simmer, partly covered, for 2 minutes. Remove meat from bones and reserve. Skim broth; return bones to pot and simmer gently for 20 minutes, partly covered. Strain and reserve stock. Discard bones.
In a heavy flameproof casserole cook onions in butter over moderate