Step 1Heat the oven to 250 degrees. Trim the meat of excess fat and any skin (fat on mature lamb can be very strong). Peel and cut 12 of the garlic cloves into slices lengthwise. Poke holes in the meat with the point of a small knife and insert the garlic slices. Tie the meat as tightly as possible with kitchen string, first across the entire length, then around the lamb at intervals of about 2 inches. Season with salt and pepper. Put it in a large, flameproof casserole with the bouquet garni, 1 teaspoon of salt and enough water to cover it by three-fourths.
Step 2Bring the water slowly to a boil on top of the stove, skimming often, 15 to 20 minutes. Cover the casserole and transfer it to the oven. Poach for 3 to 4 hours, checking every hour or so, turning the meat and adding more water if it evaporates rapidly. If the water starts to simmer, lower the heat as slow cooking is important.
Step 3Cut the leeks into 2-inch lengths, wash them and tie them in bundles of two with string. Peel and quarter the celery root and cut each piece in quarters to make 16 chunks. Season all vegetables with salt and pepper.
Step 4After 3 to 4 hours, or when the lamb is fairly tender when pierced with a two-pronged fork, lift it out and transfer the meat to a baking pan. Add the leeks, celery root, carrots, turnips and onions to the pot with the remaining 10 to 15 peeled garlic cloves and set the meat on top. If necessary add water so the leg is half covered. Continue cooking until the meat and vegetables are very tender, about 2 hours more.
Step 5To finish the dish, lift out the lamb again, place it on the baking pan, cover it with foil and set it aside. If the vegetables are not very tender, simmer them, uncovered, on top of the stove. Transfer them with a slotted spoon to a deep platter, discarding the bouquet garni and the strings for the leeks. Boil the cooking broth until well reduced and concentrated -- this may take 15 to 20 minutes. Skim the fat from the surface, taste the broth and adjust the seasoning.
Step 6If the lamb has cooled down, place it back in the casserole in the broth and reheat it on top of the stove. Discard the trussing strings from the meat and set it on the vegetables; moisten it with a little broth and serve the rest of the broth separately. You will not need to carve it as the meat will fall apart into chunks with the touch of a spoon.