In a large pot over medium high heat melt butter and cook leeks until softened.
Add stock, sherry, potato and asparagus. Bring to boil then reduce heat to medium and simmer for 15-20 minutes until all vegetables are tender.
Remove from heat and puree in blender until completely smooth. Season again to taste with salt and pepper.
Serve with swirl of cream (optional) and a Tbsp of grated Parmesan on each bowl.