In a covered saucepan, cook the onions in butter on low heat for 5-10 minutes until tender and translucent (but not browned).

Stir in the spinach and salt, then cover and cook slowly for 5 minutes or until the spinach is tender and wilted.

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

Remove from heat, then gradually mix in the boiling stock. Return to low heat and simmer for 5 minutes.

In a mixing bowl, blend the egg yolks with the cream. Slowly whisk in a ladleful of hot soup to temper the mixture, then gradually pour the egg-cream mixture back into the soup, whisking constantly.

Heat gently for a few minutes, stirring to poach the egg yolks, but do not allow the soup to boil.

Off the heat, stir in the enrichment butter one tablespoon at a time for a velvety finish.