Cook the onions slowly in the butter in a covered 2 ½ quart saucepan for 5-10 minutes, until tender but not browned. Stir in the water-cress and salt, cover and cook for about 5 minutes, or until the leaves are wilted. Sprinkle in the flour and stir over medium heat for about 3 minutes.
Off heat, beat in the boiling stock. Simmer for 5 minutes, then puree through a food mill. Return to saucepan and correct seasoning. (If not to be served immediately, set aside un-covered. Reheat to simmer before proceeding.)
Blend the yolks and cream in a bowl. Beat a cupful of hot soup in a thin stream. Teurn soup to saucepan and stir over moderate heat for a minute or two to poach the egg yolks, but avoid bringing soup to simmer. Off heat, stir in butter for enrichment, a tablespoon at a time
Origin: adapted from Julia Child's The Art of French Cooking.