Preheat the oven to 325°F degrees.
In a bowl, mix flour, salt, and ground pepper. Dredge the veal shanks in this mixture, and shake off the excess flour.
Put the olive oil in a shallow pot where you can simultaneously fit all the veal shanks. Heat on a medium/high setting on the stove. When the oil is heated, add the veal shanks and sear them until they're browned on all sides, about 4-5 minutes per side. Remove the shanks from the pot and set them aside.
Reduce the heat to medium and put butter in the pot. Add the diced onion, leek, carrot, and celery when the butter melts. Sauté until the vegetables are softened and pale in color.
Add the minced garlic and sauté for another minute.
Pour in the wine and let it simmer for a couple of minutes.
Stir in the chicken stock, thyme, dried rosemary, and lemon zest. Bring the mixture to a simmer.
Return the seared veal shanks to the pot, nestling them into the sauce.
Cover the pot with a lid and transfer it to the preheated oven. Let the Osso Buco braise in the oven for about 2 hours or until the meat is tender and easily pulls away from the bone.
Take the pot out of the oven and let it rest at room temperature for 30 minutes before serving.
Serve the dish on individual plates, spooning the sauce and vegetables over the shanks.
Garnish with freshly chopped parsley.