Preheat oven to 120°C/300°F.
Place the lamb rumps in a large ovenproof dish and score the fat. Generously spoon over about 200g/7oz of the red capsicum pesto. Place on the lowest rack in oven and bake for at least 90 minutes or until cooked to your liking. Remove from oven and allow to rest 10-15 minutes before carving. Use lamb pan juices to drizzle over sliced lamb and serve with caramelised leek, cashew and feta couscous and mint sauce.
For the caramelised leek, cashew and feta couscous, in a large bowl combine couscous and 2¼ cups of boiling water; leave for 5 minutes or until all the water has been absorbed. Using a fork, fluff the grains of couscous to help separate them.
Heat a frying pan over high heat and add leeks, onion and butter stirring until softened and beginning to brown. Turn down heat a little, add brown sugar, balsamic vinegar and cashews and continue stirring another minute or two until caramelised. Add to couscous with feta, grape tomatoes, undrained tomato strips, remaining pesto and season generously with salt and pepper. Stir to combine, garnish with fresh mint and serve with lamb.