Heat the oil in a large braiser set over medium-high heat. Dry the lamb very well and sprinkle with 1 1/4 teaspoons of the salt. Dredge each piece in the flour and then shake off the excess. Add the lamb to the pan and sear on all sides until deep golden brown, 8 to 10 minutes. Remove the lamb to a plate. Add the garlic, celery, carrots and onions to the pan and season with the remaining 1/2 teaspoon salt. Cook until the vegetables begin to brown and the mixture is fragrant, about 3 minutes. Add the tomato paste and cook, stirring often, to caramelize the paste, about 2 minutes. Deglaze with the wine and bring to a boil, then reduce the heat to medium and simmer for 3 minutes. Stir in the broth, thyme, bay leaf and rosemary. Nestle the lamb back in the pan and cover with the lid. Reduce the heat to low and cook until the lamb is soft and tender, 2 hours.
Remove the meat to a plate and keep warm. Skim the fat from the surface of the sauce and remove the herb sprigs and bay leaf. Return the meat to the sauce and serve sprinkled with the parsley and lemon zest.
Recipe courtesy of Giada De Laurentiis