In a large saucepan, heat 1/4 cup olive oil over medium-low heat. Add the sliced onion, garlic and 1/2 teaspoon salt and cook, stirring, for 10 minutes. Add the tomatoes and their juices and simmer until thickened, about 25 minutes.
Meanwhile, preheat the oven to 350 degrees . In a large dutch oven, heat 5 tablespoons olive oil over high heat. Season the lamb with salt and pepper, add to the pot and cook until browned, 15 minutes; transfer to a plate. Remove the pot from the heat and wipe clean.
Transfer the tomato sauce to the dutch oven and stir in the broth and wine; bring to a boil. Add the lamb shanks and any juices, making sure the lamb is submerged. Cover the pot, transfer to the oven and braise until the meat is falling off the bone, about 2 hours.
During the last 10 minutes of cooking, in a saucepan, heat the remaining 2 tablespoons olive oil over medium heat. Add the chopped onion and cook until softened, about 7 minutes; add 1 1/2 cups water and bring to a boil. Stir in the couscous and 1/2 teaspoon salt. Cover, remove from the heat and let stand for 5 minutes; fluff with a fork and season to taste.
Transfer the couscous to a platter and top with the lamb shanks. Skim the oil from the sauce and spoon the sauce over the lamb; sprinkle with the orange peel and mint.