Step 1Preheat oven to 180°C. Place the lamb and 1/2 cup of water in a baking dish. Pour half the lemon juice over lamb, then sprinkle with oregano. Cover dish with a sheet of baking paper and a sheet of foil; bake for 1 hour.
Step 2Line a large baking tray with baking paper. Place carrots on the prepared tray. Scatter over lemon zest. Spray carrots with oil and season with cracked black pepper. Bake for 30 minutes. Add broccoli to tray; spray with oil. Bake a further 20–30 minutes.
Step 3Increase oven temperature to 200°C. Uncover the lamb and discard foil. Bake for 50–60 minutes. Remove lamb from oven. Rest, covered with foil, for 10 minutes.
Step 4Meanwhile, make mash: Place potatoes in a large saucepan of water over high heat. Boil for 15 minutes, or until tender when pierced with a small knife. Drain and return to pan. Using a potato masher, mash the potato until almost smooth. Add garlic, half of the olive oil and lemon juice; mash until creamy. Add remaining oil and lemon; mix well. Season garlic potato mash with cracked black pepper.
Step 5Serve the roast lamb with vegetables and garlic mash.