Brown the flour in the melted butter, trickle, stirring, the hot fish bouillon into it. Boil for 40—50 minutes and strain. Separately heat the chopped onion and parsley in a little butter without letting their colour change. Join the ingredients, salt and pepper the sauce, sprinkle it with the lemon juice, simmer for 5—7 minutes, and serve with boiled fish. You can add chopped eggs and parsley to the sauce.