In a pot heat the butter and saute the onion until translucent.

Add the leeks.Saute for another minute.

Add the potatoes, milk, water, salt and pepper.

Once it comes to a boil, cover , reduce the heat and continue to cook for about 15-20 minutes until the potatoes are fully cooked.

Check the seasonings.

Carefully transfer into a blender or use a hand blender and blend until all smooth.

Garnish with your favorite herb and serve hot.