Step 1

Preheat oven to 180°C. Combine fish sauce, sweet chilli, mint and 1 tablespoon oil in a bowl. Place lamb on a rack in a roasting pan and brush with marinade. Fill pan with 2cm hot water, then loosely cover lamb with foil. Roast for 1 hour, then remove foil and roast for a further 45 minutes for pink or until cooked to your liking. Rest, loosely covered with foil, for 15 minutes.

Step 2

Meanwhile, place potatoes in a pan of cold salted water, then bring to the boil. Reduce heat to medium-high and simmer for 2 minutes. Drain well, then return to pan and shake to fluff up the edges. Transfer to a baking dish and toss with remaining oil. Season. Once lamb has been cooking for 1 hour, place potatoes in oven with lamb and roast for 45 minutes. While lamb is resting, increase oven to 220°C and roast for 15 minutes until crisp and golden.

Step 3

For the sauce, combine all ingredients in a bowl, stirring to dissolve the sugar.

Step 4

Carve the lamb and serve with roast potatoes and the mint sauce.