Trim any fat and skin off the lamb and weigh the meat to calculate the cooking time: it will take 35 minutes per kg.
Place the lamb in a stockpot and add 750 ml of water and the bay leaves, cloves, carrot, onion and peppercorns. Cover and slowly bring to the boil, then lower the heat so the liquid just simmers. Poach the meat, beginning the timing from this point. Skim off any foam as necessary during poaching.
Remove the lamb from the stock and leave it in a warm place to stand. Boil the stock hard until it has reduced by half.
Meanwhile, make the sauce. Strip the mint leaves from the stems. Place the parsley (tender stalks and leaves) into the bowl of a food processor with the mint leaves, lemon zest and juice, oil, capers, mustard and garlic, if using. Process for just long enough to chop the leaves.
Add 6 tablespoons of the reduced stock to the processed mixture and blend until it forms a reasonably smooth sauce. Season to taste then gently reheat it. (The remainder of the stock can be frozen for another use.)
Slice the lamb, garnish with a bay leaf and serve with the sauce.