Place the lamb in a large non-metallic dish and sprinkle the lemon rind and half the juice, the rosemary and the garlic slivers on top. Drizzle over half the olive oil and then turn the lamb over a couple of times until the marinade ingredients are evenly distributed. Cover with clingfilm and set aside for 2 hours to allow the flavours to combine or overnight in the fridge is even better.
Preheat the oven to Gas mark 7, 220ºC(450ºF). If the lamb has been chilled overnight, bring it back to room temperature. Then place it skin side up, in a large roasting tin and season with salt and black pepper. Roast the lamb for 20 minutes, then turn it over and roast for another 15 minutes for medium rare.
To make the salad, place the tomatoes, cucumber and red onion in a large bowl. Then tear in the mint leaves. Season to taste and then dress with the remaining olive oil and lemon juice.
When the lamb is cooked, transfer to a warm dish, then leave to rest in a warm place for 10 minutes. Carve into slices and arrange on plates with the tomato and cucumber salad and steam new potatoes.