Preheat oven to 200°C or 180°C fan. Weigh lamb and calculate cooking time (allow 20–25 minutes per 500g). Using a sharp knife, cut small slits into lamb. Insert a rosemary spike into each slit.
Scatter lemon and onion over base of a roasting pan. Sprinkle with any remaining rosemary. Place lamb on top of vegetables (or use a roasting rack). Pour 1 cup water around vegetables. Rub lamb with oil and season well.
Roast for 20 minutes. Reduce heat to 180°C or 160°C fan and roast for another 1 ¼ hours, until cooked to your liking. Cover loosely with foil and set aside to rest in a warm place for 15 minutes before carving.
Meanwhile, prepare mint sauce. Combine white wine vinegar, sugar and ¼ cup water in a saucepan on medium heat. Bring to boil and simmer for 2 minutes. Stir through mint. Serve warm or cold.
Serve lamb carved into slices with mint sauce, roast vegetables and gravy, if you like.