Mix the coriander, fennel, and peppercorns in a small frying pan. Toast on medium-high heat until aromatic and slightly darker, about 2 minutes. Using a mortar and pestle or a spice grinder, crush the spices. Add the salt. Rub each shank with the spice blend.
Preheat the oven to 325 degrees F.
Heat the oil in a large oven-proof skillet over medium-high heat. Add the shanks to the skillet. Cook until brown on all sides, about 5 minutes. Add the onion, carrots, celery, and leek. Saute over medium heat until the vegetables begin to soften, about 5 minutes. Add the port, veal stock, garlic, rosemary, and thyme to the skillet.
Cover the skillet with a lid or foil. Place the skillet in the oven and braise the lamb until tender, for 2 to 2 1/2 hours or until the meat falls off the bone. Remove the lamb shanks from the skillet and filter the sauce through a fine sieve over a stockpot. On medium heat, let the sauce reduce by 1/2, for about 15 minutes.
Meanwhile, in another skillet, melt the butter and add the lamb shank. Coat them with honey. Caramelize the shanks on all sides over medium heat on the stove for about 5 minutes, adding a ladle of the sauce. Keep warm.
Serve the lamb shanks with the sauce, buttered green beans and a parsnips mash or any other mash to taste.
NotesCan be made 1 day ahead. Let the lamb cool then place in the refrigerator covered. Reheat in a 350 degrees F oven for 1/2 hour before serving.