2 tablespoons (1/4 stick) butter
1 large onion, finely chopped
1 large cauliflower, broken into florets (about 5 cups)
2 14 1/2-ounce cans chicken or vegetable broth
1 tsp. extra-virgin olive oil
1 tablespoon thinly sliced chives
Melt butter in heavy large pot over low heat. Add onion and sauté until tender, about 10 minutes. Add cauliflower and sauté 2 minutes. Add broth. Cover and simmer until cauliflower is tender, about 25 minutes.
Working in batches, transfer soup to blender and purée until smooth. Return soup to pot. (Soup can be prepared 1 day ahead. Cool slightly. Cover and refrigerate.) Bring soup to simmer. Season to taste with salt and pepper.
Ladle soup into bowls. Drizzle with olive oil. Garnish with chives. .