Preheat the oven to 180C
Prepare the lamb by using a sharp knife to loosen the meat a little from the bone. Season with salt and pepper.
Heat the oil in a large ovenproof casserole on the hob and brown off the lamb shanks
Remove the shanks from the casserole and set aside. Turn down the heat and add the shallots, garlic, celery, carrot and bacon and fry for 3-5 minutes until soft.
Add wine and bring to the boil then add the herbs, tomato puree, stock and return to the boil.
Return the lamb shanks to the casserole, bring to a simmer, then cover and place in the oven 2-3 hours or until lamb is tender and falling off the bone.
Remove the lamb and leave to rest.
Place the casserole over a medium heat and cook until the sauce is reduce by half, skimming off any fat. Season with salt and pepper and the grated cacao.
For the cacao mash, place the potatoes into a large saucepan and cover with water. Add a pinch of salt, place on the heat and bring to the boil. Simmer for 10-15 minutes until tender.
Drain well and return to the pan over a low heat for a few seconds to remove any excess moisture.
Using a potato masher or ricer, mash well, adding the melted butter. Season with salt and freshly ground black pepper and cacao.
For the garnish, heat the olive oil in a frying pan and cook the vegetables for about 3 minutes. Then add a knob of butter.
For the gremolata combine all the ingredients.
To serve put each lamb shank on a small mound of cacao mash, pour over the sauce, add the garnish, and sprinkle over the gremolata