Heat the oven to 200C/180C fan/gas 6. Put the onions and sliced carrots in a large roasting tray. Remove any string from the lamb and put it on top of the veg, skin-side up. Pierce the lamb with the tip of a knife to make small pockets in the meat, add garlic slices and a few rosemary leaves to the holes. Rub the lamb with 1 tbsp olive oil and season. Pour 100ml water into the tray.
Roast uncovered for 45 minutes at the top of the oven. Check the internal temperature, it should read 60C for medium (pink). Cook for another 15 minutes for well done. Rest the meat under foil for 10 mins while you cook the veg.
Bring a large pan of salted water to the boil. Add the carrots, and cook for 8 minutes until tender, adding the peas and beans for the last 2 minutes. Drain the veg and toss with 1 tbsp oil, chopped herbs, lemon zest and juice. Season to taste.
Carve the lamb into thin slices. Spread the veg onto a large warmed serving platter and arrange the meat in the middle.