Add 2 teaspoons of the remaining olive oil, the onion and three-quarters of the minced garlic to the casserole. Cook over moderately low heat, stirring occasionally, until softened, 7 minutes. Add the tomato and cook over moderately high heat for 3 minutes. Stir in the wine and simmer until reduced by half. Return the lamb shanks to the casserole, nestling them in an even layer. Add the water and bouquet garni and bring to a gentle simmer. Cover the shanks with a sheet of parchment paper and the lid and bake for 1 hour. Turn the shanks, cover again with the parchment and the lid and bake for 1 hour longer, or until the meat is very tender.