1. Heat oil in a 5- to 6-quart heavy pot over moderately high heat until hot but not smoking and brown lamb in batches, about 6 minutes per batch. Transfer lamb with a slotted spoon to a bowl, reserving pot.
2. Add onions to pot and cook over moderately high heat, stirring, until golden, about 5 minutes. Add broth and 1 cup water, scraping up browned bits from bottom of pot. Arrange sliced russet potatoes over onions, slightly overlapping them, and sprinkle with half of thyme and chives. Return lamb to pot and tuck halved new potatoes between cubes of meat, pressing both into liquid. Sprinkle with remaining thyme and chives. Bring liquid to a boil and cook at a bare simmer, covered (do not boil), until meat and potatoes are tender, about 2 hours.
3. Serve stew in bowls with crusty bread for dipping.