In a well-buttered heavy saucepan combine the fish bones and trimmings, the onion, the parsley, the lemon juice, and the salt and steam the mixture, covered, over moderately high heat for 5 minutes. Add the water and the wine, bring the liquid to a boil, skimming the froth, and simmer the mixture for 20 minutes. Strain the stock through a fine sieve into a bowl, let it cool to warm, and chill it, covered. The stock keeps, covered and chilled, for 1 week if it is brought to a boil every 2 days and then allowed to cool to warm, uncovered, before being chilled again. The stock keeps, frozen, for 3 months. Makes about 3 cups.