Preheat the oven to 200°C (400°F/Gas 6). Rinse the lamb and pat dry with paper towels. Cut 5 or 6 long slashes on the top. Make a bed of the fennel branches on the bottom of an oven dish and drizzle 4 tablespoons of olive oil over. Salt and pepper both sides of the lamb well and rub in the crushed fennel. Press a garlic half, a rosemary sprig, a sage leaf and a few slices of tomato into each slash. Put the lamb on top of the fennel branches and drizzle the remaining 2 tablespoons of olive oil over.
Roast until the top of the lamb is starting to change colour, about 20 minutes. Pour in the wine and add the potatoes around the lamb, turning them through the liquid. Lower the temperature to 180°C (350°F/Gas 4) and roast for 1 hour 15 minutes, turning the potatoes a few times. The lamb should be golden, nicely cooked and soft, and the potatoes should be golden brown. Serve the lamb cut in thick chunks, with the potatoes.