Preheat oven 200°C. Place lamb in a large shallow roasting pan. Drizzle with half the oil. Season to taste. Roast for 15 minutes.
Meanwhile, place potatoes in a large saucepan and cover with cold water. Bring to the boil over high heat and cook for 15 minutes or until just tender. Drain. Cut each potato in half.
Remove lamb from oven. Add potatoes. Using a potato masher, crush potatoes. Drizzle with remaining oil and season to taste. Reduce heat to 180°C. Return lamb and potatoes to oven and cook for 1 hour for medium or until lamb is cooked to your liking and potatoes are crispy.
Meanwhile, bring a saucepan of water to the boil over high heat. Add 2 mint sprigs and peas. Cook for 2-3 minutes or until bright green and tender. Drain. Remove and discard mint sprigs.
Remove mint leaves from remaining mint sprigs. Reserve 10 leaves and finely chop remaining. Combine yoghurt, chopped mint and lemon juice in a small serving bowl. Season to taste. Serve lamb and potatoes with yoghurt mixture and peas. Sprinkle with reserved mint leaves, to garnish.