Begin by heating the butter in a large Dutch oven over medium heat until it melts and begins to crackle softly.
Add the sliced leeks to the pot and cook gently, stirring occasionally, for about 8-10 minutes until they become translucent and fragrant, filling your kitchen with a sweet, mellow aroma.
Stir in the diced potatoes and cook for another 2 minutes, allowing their edges to soften slightly and develop a subtle toasted note.
Pour in the vegetable stock, bringing the mixture to a gentle simmer over low-medium heat, and cook uncovered for 20-25 minutes until the potatoes are tender and easily mashable.
Use an immersion blender to blend the soup directly in the pot, or carefully transfer in batches to a blender, blending until the mixture is completely smooth and silky, with no lumps remaining.
Return the blended soup to gentle heat, stir in the heavy cream, and season with salt, pepper, and a pinch of nutmeg, simmering for another 5 minutes to allow flavors to meld and the soup to warm through.
Taste the soup and adjust the seasoning as needed, then remove from heat. Ladle into bowls, garnish with chopped fresh herbs if desired, and serve immediately while steaming hot.
