Melt butter in saucepan over medium heat. Add onions and leeks, saute gently until soft. Stir in flour and cook for 2 minutes. Gradually stir in stock.

Add potatoes, bring to a boil. Reduce heat and simmer gently for 30mins. or until potatoes are soft. Stir in milk and seasonings.

Puree soup in a blender or food mill. Add salt and pepper to taste.

Top each serving with chopped chives and creme frache.