Melt the butter in a heavy bottomed pot over medium heat - I use my Le Creuset for this. Add the onion and leeks and reduce the heat to medium-low. Put a lid on the pot and cook the onions and leeks until soft - about 15 minutes. At this level of heat, it's more sweating than sautéing - be careful not to brown!

Add the potatoes and garlic. Replace the lid to the pot and cook for another 5 minutes.

Add the broth and watercress, and bring to barely a simmer. Cook for 5 minutes, and pour the soup into a blender. I use my Ninja blender for this. Purée the soup and pass through a food mill if desired. The food mill will ensure that any stems remaining are removed.

Stir in the buttermilk, salt and pepper. The soup may taste under salted, but when chilled it will be fine. Chill for a few hours or ideally overnight before serving.

Whip together the mascarpone cheese, salt and lime juice with a small whisk or fork. Drop a ½ teaspoon dollop of cheese on each soup shooter.