KALE AND CHICKPEA SOUP

1 medium onion, chopped (1 cup)
2 garlic cloves, chopped
1 Turkish or 1/2 California bay leaf
1 teaspoon salt
1/8 teaspoon black pepper
2 tablespoons extra-virgin olive oil
1 large boiling potato (3/4 lb), peeled and cut into 1/2-inch pieces
3/4 lb kale, stems and center ribs cut out and discarded, then leaves very finely chopped (4 cups)
3 1/2 cups chicken or vegetable broth (28 fl oz)
2 cups water
1 (14-oz) can chickpeas, rinsed and drained
1/4 lb Spanish chorizo (optional), cut into 1/4-inch dice (1 cup) 

Cook onion, garlic, bay leaf, salt, and pepper in oil in a wide 4- to 6-quart heavy pot over moderate heat, stirring frequently, until onion and garlic are softened and beginning to brown, 5 to 7 minutes. Add potato, kale, broth, and water and cook, partially covered, until potatoes are tender, 15 to 20 minutes. Reduce heat to low, then add chickpeas and chorizo and gently simmer, uncovered, 3 minutes. Discard bay leaf and season with salt and pepper.