Step 1Heat the oven to 250 degrees. Trim the lamb of excess fat and any skin. Tie the meat as tightly as possible with kitchen string, first across the length of the meat and then around the meat at intervals of about 2 inches. Put it in a large, oval flameproof casserole and pour the Cognac over the lamb, reserving a tablespoon. Warm the lamb on the stove over moderate heat, and light it with a long match, standing back as the flames rise high. When they die after about 2 to 3 minutes, remove the lamb to a baking pan, season it with salt and pepper and set the casserole aside. Blanch the bacon by putting the strips in a saucepan of cold water. Bring the bacon to a boil, simmer 5 minutes and drain. Peel and quarter the celery root and cut each piece in quarters to make 16 chunks.
Step 2Set the bacon on the bottom of the casserole and spread the celery root, carrots, turnips, onions and garlic cloves on top. Add the lamb with the bouquet garni, wine, a pinch of salt and enough water to cover the meat by three-fourths. Bring the casserole slowly to a boil on top of the stove, skimming often, for 20 to 25 minutes. Cover and transfer it to the oven. Poach the lamb until the vegetables are very tender, with the meat almost falling from the bone, 5 to 6 hours. Check every hour or so, turning the lamb and adding more water if it evaporates rapidly. If the water starts to simmer, reduce the heat.
Step 3To finish the dish, transfer the lamb to a baking pan, cover it with foil and set it aside to keep warm. Discard the bouquet garni. Drain the bacon and vegetables in a colander, reserving the cooking liquid. Skim off the fat. Separate the bacon, dice it and set it aside. Return the vegetables to the casserole, add about half the reserved liquid and puree the mixture, coarsely or finely as you prefer, with a stick blender (or you can puree the vegetables with a bit of liquid in a food processor and return the puree to the casserole). Stir in the bacon dice, with more liquid if needed so the puree just falls easily from a spoon. Stir in the reserved Cognac, taste and adjust the seasoning.
Step 4Warm the puree in the casserole on top of the stove. Discard strings from the meat, set it on a deep platter and spoon the puree around the edge. Reheat the remaining cooking liquid; serve with the lamb. You won't need to carve the meat as it will fall apart in chunks with the touch of a spoon.