Scrub the salsify in plenty of water, discard the ends of the roots and cut into 1 cm ( half in) lengths. Soften the onions with the salsify in the sunflower oil for 10 ... 15 minutes in a heavy, covered pan. Stir occasionally to prevent sticking, as salsify is dry. Meanwhile stew the mushrooms in the olive oil until they have given off their liquid, reserve the liquid and add the mushrooms to the salsify and onions. Cover with the water and bring to the boil; simmer for 20 minutes. In a heavy pan, melt the butter, stir in the flour and gradually add first the mushrooms-liquid, then the milk. When you have a thick sauce, slowly stir in the liquid from the soup, add the vegetables and liquidize. Season with salt and pepper to taste. (Salsify is a long, thin, darkly earth-coloured root, can be found in greengrocers's shops in February, also available as preserved ; when scrubbed, it is revealed to be white whithout an obvious outer skin. Also known as "vegetable oysters" ~ some curiously salt, fishy flavour or as "poor people's asparagus")