Trim the shanks, and season with salt and pepper.
In a heavy bottom Dutch oven, heat the oil over medium heat and brown the shanks evenly on each side until golden brown.
Remove the shanks to a plate, then add the onion, celery, carrots, and garlic, and cook until just beginning to brown, about 5 minutes.
Add the rosemary, oregano, wine, and stock to the pot and bring to a boil.
Return the shanks to the pot, cover, and reduce to a simmer.
Cook for 1 ½ hours, turning the shanks over occasionally, until the meat falls off the bone.
While the meat is cooking, place the parsley, garlic, and lemon zest on a cutting board, and coarsely chop.
Place the shanks on a warm platter, and remove the rosemary sprig.
Drain off any visible fat from the remaining liquid, then puree it with an immersion blender (hand wand).
Taste, and adjust salt and pepper as needed.
Place the polenta or potatoes on a platter and spoon the sauce around the potatoes or polenta.
Carve the shank into large chunks and arrange on top of the potatoes or polenta.
Sprinkle the shanks with the gremolata, then serve immediately.
