Heat half the oil in a large, heavy, covered casserole or roasting pan, large enough to hold the lamb shanks. Add the lamb shanks, a few at a time, and brown them well on all sides over medium-high heat. Remove from the pan.
Preheat the oven to 350 degrees.
Add the remaining oil to the pan, lower the heat and saute the onion, leeks and celery until they are soft and lightly browned. Stir in all but 1/2 tsp of the garlic, then add the wine. Simmer for a few minutes, scraping the browned bits from the bottom of the pan.
Stir in the rosemary, parsley sprigs and bay leaves. Season the mixture with salt and pepper, then return the lamb to the pan. Cover and place in oven to bake until the lamb is very tender, about 3 hours.
While the lamb is baking, mix the remaining 1/2 tsp garlic with the minced parsley and lemon peel and set aside.
When the lamb is tender, remove it from the pan. Bring the liquid to simmer on top of the stove and taste it for seasoning, adding more salt and pepper, if necessary. Dissolve the potato starch in the cold water and stir it in to thicken the sauce, then stir in the lemon juice.
Return the lamb to the pan and baste it with the sauce. Can be prepared in advance and reheated before serving. Transfer the lamb to a platter and spoon the sauce over it. Sprinkle with the minced parsley, garlic and lemon peel mixture and serve.