1.In a bowl combine oil, rosemary, cumin, zest, garlic, salt, pepper and chilli.
2.Place lamb in a shallow dish, pour marinade over, turning to coat. Chill, covered, for 2 hours.
3.Heat a large frying pan on high. Cook drained lamb in 2 batches, for 4-5 minutes each side, until cooked to taste. Cover with foil and rest for 5 minutes.
4.In a small bowl, combine yoghurt and mint. Season to taste. Keep chilled.
5.To make watermelon salad, combine watermelon, feta, olives and mint in a large bowl. In a jug, whisk together oil and lemon juice. Season to taste and drizzle over salad, tossing well.
6.Slice lamb and serve with salad and minted yoghurt.
Boneless lamb leg can be used – bake in a moderate oven for 20 minutes per 500g.