Pistou is just a French version of pesto.
In a large saucepan, melt butter over medium. Add onion, leek, carrot and garlic and lightly cook vegetables without colouring, about 2 minutes. Add wine and reduce by half, about 2 minutes. Add potatoes, asparagus stems, and stock. Bring to a boil. Reduce heat, partially cover and let simmer until potatoes and asparagus stems are tender, about 15 to 20 minutes.
Steam asparagus tips. (Alternatively place a mesh strainer into the soup broth and add the asparagus tips to cook until crisp tender about 2 minutes.) Set the tips aside until ready to use.
Add the chopped spinach to the soup and cook for 1 minute.
Carefully puree soup in a blender or with a hand blender until smooth. Note if soup seems stringy from the asparagus, strain soup through a mesh strainer.) Stir in buttermilk. Reheat until steaming. Season with cayenne, salt and pepper. Serve and garnish with asparagus tips and pitsou.
Combine all ingredients in a food processor and pulse until smooth. Garnish soup with a dollop of the pistou.