Mince green or yellow onions.

Cook onions slowly in butter in a covered saucepan for 5-10 minutes until tender and translucent, but not browned.

Add spinach (or watercress) and salt.

Cover and cook slowly for about 5 minutes until leaves are tender and wilted.

Sprinkle in flour and stir over moderate heat for 3 minutes.

Off heat, beat in boiling stock.

Simmer for 5 minutes, then puree through a food mill.

Return to saucepan and correct seasoning.

If not serving immediately, set aside uncovered. Reheat to simmer before proceeding.

Blend egg yolks and cream in a mixing bowl.

Beat a cupful of hot soup into the egg yolk mixture by driblets.

Gradually beat in the rest of the soup in a thin stream.

Return soup to saucepan and stir over moderate heat for a minute or two to poach the egg yolks, but do not bring the soup to a simmer.

Off heat, stir in softened butter a tablespoon at a time.

Pour the soup into a tureen or soup cups and decorate with optional watercress leaves.

To serve cold: Omit final butter enrichment and chill. If too thick, stir in more cream before serving.