Prepare the leeks: Cut off the dark green tops and roots of the leeks. Slice the remaining white and light green parts in half lengthwise, then rinse thoroughly under cold water to remove any grit between the layers. Slice into thin half-moons.

Sauté the aromatics: In a large pot over medium heat, melt the butter. Add the leeks and onion, season lightly with salt, and cook gently for 8-10 minutes until softened but not browned. Stir occasionally.

Add potatoes and broth: Add the potato chunks and broth to the pot. Bring to a gentle boil, then reduce heat and simmer, partially covered, for about 15-20 minutes or until the potatoes are very tender when pierced with a fork.

Blend the soup: Remove from heat and let cool slightly. Purée the soup using an immersion blender, or transfer in batches to a regular blender until completely smooth.

Strain for silkiness: For the most refined texture, pass the puréed soup through a fine-mesh sieve into a clean pot or bowl, pressing gently with the back of a ladle.

Add dairy: Stir in the heavy cream and milk. Season to taste with salt and white pepper. If using, add a small pinch of nutmeg.

Chill thoroughly: Cover and refrigerate for at least 30 minutes, though 2-3 hours or overnight is best for the flavors to fully develop.

Serve: Ladle the chilled vichyssoise into bowls and garnish with freshly snipped chives. For an extra elegant touch, add a small swirl of cream on top.