Make the sauce by softening the onions in the butter. Stir in the flour and tomato purée and blend in the stock or water slowly in order to avoid lumps. Season. Bring the sauce to the boil and leave it to simmer very gently for about 20 minutes.
Add the precooked salsify to the sauce and allow to sit in it for 5 minutes or so but do not continue cooking too long as the salsify should re