Gently sweat the chopped leeks and the chopped onion in butter or margarine until soft, about 8 minutes. Do NOT let them brown.Advertisement
Add potatoes and stock to the saucepan. Salt and pepper to taste; do not overdo them! Bring to the boil, and simmer very gently for 30 minutes.
Puree in a blender or food processor until very smooth. Cool. Gently stir in the cream before serving.