Preheat oven to 190C/170C fan-forced.

Pat lamb dry with paper towel. Combine rosemary, garlic and oil in a small bowl. Season with salt and pepper. Rub mixture all over the lamb, ensuring you get it right into all those crevices.

Place lamb in a flameproof roasting pan. Cover tightly with foil or a tight-fitting lid. Roast for 2 hours 30 minutes or until the meat can be pulled off the bone using a fork. Remove foil or lid. Roast for a further 15 minutes to brown the skin.

Meanwhile, make mashed potato. Bring a large saucepan of water to the boil. Add potato. Cook for 8 minutes or until very soft. Drain well. Return potato to the pan (do not return to the heat). Add butter and milk. Mash until smooth, adding a little extra milk to make it creamier, if needed. Season with salt and pepper. Cover to keep warm.

Transfer lamb to a serving plate. Cover loosely with foil. Rest for 15 minutes.

Make gravy by skimming fat from roasting pan, leaving 1/4 cup pan juices and fat. Place pan over medium heat. Add flour. Cook, stirring with a wooden spoon, for 1 to 2 minutes or until mixture bubbles and browns slightly. Add 1/2 cup chicken stock. Cook, stirring to scrape any cooked-on bits from the bottom of the pan. Whisk in remaining stock and 1 cup water. Increase heat to medium-high. Whisk constantly until mixture simmers and thickens. Season with salt and pepper. Strain gravy through a sieve into a jug (if desired). Serve lamb with mashed potato, peas and gravy.