Peel and halve the onions, cut them into delicate rings. Melt 1 tbsp. margarine in a big pot, add the onion rings and cook. Once the onions are translucent, add flour. Stir for approximately 1 minute until thickened. Bit by bit add the hot water to the pot. Allow the mixture to cook for 20 minutes.

In the meantime cube the piece of toast. Melt the remaining tablespoon of margarine in a skillet. Add the cubed toast and roast until golden-brown.

In a small bowl combine the egg yolk and heavy whipping cream. Take the pot from the stove and add this mixture. Add salt and pepper to taste.

Serve the soup in bowls, adding the bread cubes to each bowl. Sprinkle chives or parsley, if desired.