Watercress-Cauliflower Soup

2 Tbsp unsalted butter
1 small onion, coarsely chopped
1 small head cauliflower, cut into 1" florets
2 c homemade or low-sodium chicken or vege broth
2 1/2 tsp coarse salt
Freshly ground pepper
1 bunch watercress, thick stems removed + small sprigs for garnish

Melt butter in a med saucepan over med heat. Add onion; cook, stirring occasionally, until soft and translucent, about 5 min. Add cauliflower, stock, salt, 1/4 tsp pepper, and 2 c water. Bring to a boil. Cover and reduce heat; simmer, stirring once, until cauliflower is very tender, about 15 min. Remove from heat, and stir in watercress.

Working in batches, puree soup in blender, filling no more than 1/2 way. Return soup to saucepan; cover to keep warm. To serve, divide among 4 bowls. Garnish each bowl with a sprig of watercress, and season with pepper.