If you have time to marinate the lamb overnight it will improve the flavour and texture of the meat, but even an hour will make a difference. In a blender, make a marinade with the wine or sherry, oil, capers, mustard, anchovies, parsley, garlic and pepper. With a sharp knife, make slashes in the lamb every 5–6 cm about 1 cm deep. Massage the marinade into the lamb, put into a dish or baking tray, cover with foil and leave in a cool place.
When ready to cook, preheat the oven to 190°C/Gas 5. Put the chopped onion in a small mound at the bottom of the baking tray with a sprig of rosemary and the lavender if using. Put the lamb on top of the onions, which act as ‘gravy browning’. Sprinkle the rest of the rosemary and thyme on top of the meat. Put in the oven and bake for 20 minutes per 500 g, for pink, 30 for medium, or until cooked to taste. When ready, take it out of the oven and leave to stand in a warm place loosely covered with foil, for 10–15 minutes.
Make a gravy by scraping up the onion and any other juices and sediment in the tin. Take out the rosemary and add more wine if you like. When you have a thickish sauce, add the chocolate or cocoa and stir in well; it should thicken the sauce a little and give a deep colour and aroma. Strain or simply remove any very burnt pieces of onion before serving.