Prepare the vegetables: trim and slice the white and light green parts of the Leek and rinse well to remove any grit; peel and cut the Floury Potatoes into even chunks; finely dice the Onion.
Melt the Unsalted Butter in a large heavy-bottomed pot over medium heat, then add the diced onion and sliced leek and sweat gently until softened but not browned, about 6–8 minutes.
Add the prepared potato chunks to the pot, stir to combine, then pour in enough Stock to just cover the vegetables and tuck in a Bay Leaf.
Bring to a gentle simmer, cover, and cook until the potatoes are very tender when pierced with a fork, about 15–20 minutes.
Remove the bay leaf, then use an immersion blender to purée the soup until smooth (or carefully transfer in batches to a blender).
Return the blended soup to the pot and stir in the Whole Milk to loosen the texture; warm through gently but do not boil.
Finish by stirring in the Double Cream for extra silkiness, then season to taste with Salt and freshly ground Black Pepper.
Ladle into bowls and sprinkle with chopped Chives before serving. Adjust seasoning and consistency with a little more milk or stock if needed.
