Preheat oven to 220°C or 200°C fan forced. Line a large baking tray with baking paper. Place vegetables in a single layer on prepared tray, spray with olive oil and season with freshly ground black pepper. Roast for 35-40 minutes, or until vegetables are tender.
Meanwhile, brush lamb rumps with olive oil. Heat a large non stick frying pan over a high heat. Cook lamb, for 1-2 minutes each side or until browned. Transfer lamb to a rack over a roasting dish. Spread 2 teaspoons of the tahini over each lamb rump, then sprinkle evenly with the dukkah. Roast for 15-20 minutes for medium, or until 65°C on a meat thermometer. Remove, cover loosely with foil and set aside to rest for 10 minutes.
Meanwhile combine yoghurt, remaining tahini and lemon juice. Cut the lamb across the grain into slices of even thickness Divide lamb and roasted vegetables between plates, serve with the tahini yoghurt.
Dukkah is an Egyptian spice mix, look for it in the spice section of your supermarket. To make your own dukkah, combine 2 tsp toasted cumin seeds, 2 tsp toasted coriander seeds, ¼ cup toasted pistachios or almonds and 1 tbsp sesame seeds in a food processor until finely chopped.