Southwestern Corn and Potato Soup

Heat oil in a 5- to 6-quart heavy pot over moderate heat until hot but not smoking, then cook onion, jalapeño, salt, and pepper, stirring occasionally, until onion is pale golden, about 8 minutes.

Meanwhile, peel potatoes and cut into 1-inch pieces.

Add broth, water, and potatoes to onion mixture and cover pot, then bring to a boil over high heat.

Reduce heat and simmer, uncovered, stirring occasionally, until potatoes are very tender, 12 to 14 minutes.

Coarsely mash potatoes in pot with a potato masher. Stir in corn and simmer, uncovered, 3 minutes.

Stir in lime juice, cilantro, and salt to taste.