Melt the butter in a saucepan and add all the onions. Cover and cook over very low heat for
about 10 minutes or until soft.
Add the potatoes and the stock. Bring to the boil, cover again and simmer over a moderately low
heat for 30 minutes. Cool slightly.
Puree the soup in a blender or in a food processor.
If serving the soup hot, pour it back into the pan. Add cream and season. Reheat gently,
stirring frequently. Stir in the lemon juice and garnish before serving.