Melt the butter in a saucepan, and sweat the leeks and onion, stirring occasionally, for about 5 minutes.
Add the potatoes, vegetable stock, lemon juice, nutmeg, coriander, and bay leaf.
Season with salt and pepper.
Bring to the boil, cover, and simmer for 30 minutes until the vegetables are soft.
Cool the soup a little, remove the bay leaf, and purée the soup in a blender or food processor until smooth.
Pour into a clean pan.
Blend the egg yolk into the cream, add a little of the soup to the mixture, then whisk it all back into the soup and reheat gently.
Notes / Tips / Wine Advice:
Cool and chill before serving the creamy leek and potato elegance.Wine Advice:Pair this classic Vichyssoise with a crisp and dry white wine like Chardonnay or Sauvignon Blanc. The wine's acidity will complement the creamy texture of the soup, and the subtle herbal notes will harmonize with the leeks and potatoes.