Step 1

Melt butter in a medium pot over medium heat. Add onion and cook, stirring often, until softened, 5–6 minutes. Add broth and potatoes; bring to a boil over medium-high heat. Reduce heat to medium and simmer, stirring occasionally, until potatoes are tender, about 20 minutes. Remove from heat.

Step 2

Add watercress to pot; stir until leaves wilt. Let soup cool briefly. Working in batches, purée soup in a blender until smooth. Return soup to same pot. Stir in 1½ tsp. lemon juice. Season to taste with salt, pepper, and more lemon juice, if desired.

Step 3

Whisk cream with a pinch of salt in a large bowl until soft peaks form. Fold in 1½ Tbsp. chives.

Step 4

Rewarm soup gently over low heat. Divide soup among demitasse or small teacups and garnish each cup with a dollop of whipped cream. Sprinkle remaining 1 1/2 Tbsp. chives over.