Black Bean and Chorizo Soup

1 (3- to 4-oz) link of Spanish chorizo spicy cured pork sausage), coarsely chopped
1 medium onion, chopped
1 large garlic clove, finely chopped
1/2 green bell pepper, chopped
1/4 teaspoon dried hot red pepper flakes
1/2 teaspoon ground cumin
1/4 teaspoon salt
2 tablespoons olive oil
2 1/2 to 3 cups chicken broth
2 (15- to 19-oz) cans black beans, rinsed and drained

Garnish: rounds of thinly sliced lemon and chopped fresh cilantro

Cook chorizo, onion, garlic, bell pepper, pepper flakes, cumin, and salt in oil in a 3-quart heavy saucepan over moderate heat, stirring, until vegetables are softened, about 10 minutes. Add broth (2 1/2 cups for smaller cans of beans or 3 cups for larger) and beans and simmer, partially covered, 15 minutes. Lightly mash beans with a potato masher to slightly thicken.
Soup keeps, chilled and covered, 2 days.