Pound the cumin, rosemary, peppercorns and chilli in a mortar with a pestle, until coarsely ground. Add the lemon and orange zest, then stir in the oil. Place the lamb in a shallow dish, pour over the marinade, then turn to coat well. Cover and refrigerate for 2 hours.
Preheat the oven to Gas Mark 6, 200°C, fan180°C. Heat a heavy-based frying pan over a medium-high heat. In two batches, sear the lamb on all sides, turning, for 2-3 minutes. Transfer the lamb to a baking tray, then roast for 7 minutes for medium-rare, or until cooked to your liking. Set aside for 5 minutes to rest.
Meanwhile, for the watermelon salad, whisk together the oil and lemon juice, then toss in a large bowl with the remaining ingredients. Stir the mint into the yogurt and season well. Slice the lamb and serve with the watermelon salad and minted yogurt on the side.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.