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1
Heat butter in a large saucepan or Dutch oven over medium heat until melted. Add leeks, reduce heat to low, and cook, stirring frequently, until very soft but not browned, 10 to 15 minutes.
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2
Add stock, potatoes, and bay leaf, and season lightly with salt and pepper. Bring to a boil over high heat, reduce to a gentle simmer, cover, and cook until potatoes are fall-apart tender, about 15 minutes.
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3
Remove potatoes from soup using tongs and transfer to a bowl. Set aside. Discard bay leaf. Transfer remaining soup to a blender. Slowly increase blender speed to high and blend until completely smooth, about 2 minutes. Return soup to a clean pot.
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4
Press potatoes through a potato ricer or food mill into the pot with the soup. Whisk in buttermilk and heavy cream. Whisking frequently, bring soup to a simmer over medium-high heat. Whisk in grated nutmeg. Season to taste with salt and pepper and serve with chives or scallions.
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5
Add heavy cream and buttermilk to pot. Discard bay leaf. Working in batches if necessary, transfer soup to a blender. Slowly increase blender speed to high and blend until completely smooth, about 2 minutes. Return soup to a clean pot, pressing it through a fine mesh strainer or chinois with the bottom of a ladle if a smoother texture is desired. Whisking frequently, bring soup to a simmer over medium-high heat. Alternatively, chill completely and serve cold. Whisk in grated nutmeg. Season to taste with salt and pepper and serve with chives or scallions.
