Season the lamb with salt and pepper. In an 8- to 10-quart Dutch oven, heat the olive oil over medium heat until smoking. Increase the heat to medium-high, add the lamb in 2 batches and cook on all sides until golden-brown, about 8 minutes total; transfer to a bowl and set aside.
Add the pancetta to the pot and cook over medium-high heat until lightly browned, about 3 minutes; add the onion, celery, carrots and fennel seeds and cook until softened, about 6 minutes. Add the olives, wine, tomato sauce and chiles and bring to a boil. Stir in the reserved lamb pieces and bring to a boil. Lower the heat and simmer, stirring occasionally, until the lamb is tender, about 1 1/2 hours.
Skim the fat off the top of the stew, if desired; serve the stew over the polenta.