In a heavy bottomed stock pot or dutch oven, heat oil over medium high heat, add pancetta, cook until browned. Remove from pan and drain on paper towels, reserving fat in pot.
Season the lamb shanks with salt and pepper. Brown the shanks on all sides, over high heat in the reserved pot. Once they are browned, remove from pot.
Add onion, celery, carrots and garlic to the pot. Cook until softened, 3-5 minutes. Add shanks back to pot and add wine, tomatoes, rsemary, thyme, bay leaves and just enough broth to cover the meat. Bring to a boil then reduce to simmer. Cook covered, 2-3 hours (until meat is tender)
Remove lamb, rosemary sprigs, thyme and bay leaves from the pot and puree the sauce(immersion blender would be easiest). You may want to take some of the liquid out if there is too much - add back in gradually as you puree the sauce until it is the consistancy you like. Add the pancetta back in and serve shanks with the sauce over polenta or pasta.