Bring the cream and 2 tablespoons whiskey to a simmer in a medium skillet over medium heat; cook until reduced to about 3/4 cup, about 10 minutes. Stir in the remaining 1 tablespoon whiskey. Transfer to a bowl and refrigerate until cold.
Meanwhile, melt the butter in a large saucepan over medium heat. Add the onion and celery; cover and cook, stirring occasionally, until softened, about 5 minutes. Add 2 cups water, the potato, broth, sugar and 1/2 teaspoon salt; bring to a simmer and cook, partially covered, until the potato is tender, about 10 minutes.
Add the watercress to the saucepan and simmer until just tender, about 5 minutes. Transfer the soup to a blender in batches and puree until smooth. Return to the saucepan and bring to a bare simmer; season with salt and pepper.
Whisk the chilled whiskey cream until soft peaks form. Ladle the soup into bowls and top each with a dollop of the whipped cream.
Photograph by Anna Williams
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.