Preheat your oven to 230°C (450°F) / 210°C Fan / Gas Mark 8. Scrub the potatoes clean and bake them directly on the oven rack for 1 hour, or until completely tender when pierced with a knife.
Once cool enough to handle, carefully peel the potatoes with a small knife.
Set the skins aside and roughly chop the potato flesh.
To make a flavourful stock, place the reserved potato skins in a pan with 750ml (25.4 fl oz) of water.
Bring to a simmer and cook for 10 minutes.
Let it cool slightly.
In a separate large saucepan, melt the butter over medium-low heat.
Add the finely chopped onion and cook gently until soft and translucent, but not browned.
Strain the potato skin water through a sieve directly into the saucepan with the onions, discarding the skins.
Add the chopped potato flesh and bring the mixture to a gentle simmer for 10 minutes.
While the soup simmers, prepare the topping by mixing the chopped chives and sour cream in a small bowl.
Season with a pinch of salt and pepper.
Remove the soup from the heat.
Using an immersion blender, carefully blend the soup until it is completely smooth. Return the pan to low heat.
Stir in the whipping cream.
Heat through gently, but do not allow the soup to boil as the cream may split. If the soup is too thick, thin it out with a splash of whole milk until you reach your desired consistency.
Season to taste with salt and pepper.
Ladle the hot soup into bowls and top each with a generous dollop of the chive sour cream to serve.
Using whipping cream results in a lighter, more delicate texture than using double or heavy cream.
Turn this soup into a hearty meal by serving it with a side of cheesy Leek and Cheddar Welsh rarebit.
