To a bowl add the sugar and boiling water. Mix until dissolved. Stir in the cold water, vinegar and mint. Set aside.
Make the lamb:
Heat the oven to 425°F/220°C with the shelf in the lower part of the oven.
Drizzle 2 tablespoons oil all over the lamb and rub with hand to even coat. Sprinkle evenly all over with salt and pepper.
Use a sharp tipped knife to make 8 cuts at a slight angle evenly all over the top of lamb, just deep enough for the garlic cloves. Insert garlic and rosemary sprigs into each hole.
Drizzle the bottom of a deep roasting pan with 1 tablespoon oil and spread evenly. Lay the potato slices in the bottom of the pan. Drizzle the rest of the oil on top of the potatoes and sprinkle salt, pepper and chopped rosemary. Top with the fennel and onion.
Place the lamb on top of the vegetables and roast for 20 minutes. Lower the oven temperature to 375°F/190°C and roast for 25 minutes per pound (per 500 grams) plus 25 more minutes for medium temperature (slightly pink). Best measured with a meat thermometer. 150°F/65°C for well done, 145°F/62°C medium well, 130°F/55°C for medium. 125°F/52°C for medium rare. Times, will be longer if your lamb is bone-in.
Remove from the oven and cover with foil for 10 minutes before slicing.