Preheat oven to 200°C. Cut six slits, 2cm deep and 2cm long, into the lamb. Press one piece of garlic into each slit. Place the lamb in a large roasting pan. Sprinkle evenly with rosemary and drizzle over 1 tablespoon of oil. Season with salt and pepper. Roast in oven For 15 minutes.
Remove from oven and place the potato, in a single layer, around the lamb. Drizzle the remaining oil over the potato. Season with salt and pepper. Roast in oven for a further 1 hour for medium or until cooked to your liking.
Meanwhile, cook peas in a saucepan of boiling water for 3-4 minutes or until just tender. Drain and return to pan. Add the butter and mint and toss to combine.
Transfer the lamb to a plate and cover with foil. Set aside for 10 minutes to rest. Turn potato and roast in oven for a further 10 minutes or until crisp. Transfer to a bowl. Cover with foil to keep warm.
To make the gravy, place the roasting pan over high heat. Add the flour and cook, stirring, for 1 minute or until the mixture bubbles. Gradually add the stock and wine, and cook, scraping the pan to dislodge any bits that have cooked onto the base, for 5-8 minutes or until the gravy thickens.
Thickly slice the lamb across the grain. Serve with potato, peas and gravy.