If the sweet potatoes are not already roasted, preheat your oven to 220°C / 425°F. Prick the peeled sweet potatoes with a fork, place on a baking sheet and roast for 35–45 minutes until very tender. Roasting concentrates their natural sugars and gives the mash a deeper flavor than boiling. If they're already roasted, skip this step and let them cool slightly before mashing.

Bring the lamb to room temperature for 30 minutes before cooking (take it out of the fridge). Pat it dry with paper towels. This step evens the cooking so the center won’t be cold and helps the exterior brown better; moisture on the surface will steam rather than sear.

Make the herb paste: in a small bowl combine the chopped rosemary, minced garlic, 2 tbsp olive oil, 1 tsp kosher salt and 1/2 tsp freshly ground black pepper. Mix into a coarse paste. The oil helps the herbs and garlic adhere and releases aromatic oils that flavor the crust while roasting.

Rub the paste evenly all over the lamb, pressing it into any crevices. For best flavor, cover and refrigerate for at least 30 minutes or up to overnight. Marinating lets the rosemary and garlic infuse the surface of the meat — overnight develops more flavor, but 30 minutes at room temperature will still result in a great crust.

Lower the oven to 200°C / 400°F. Toss the cut carrots and parsnips with 1 tbsp olive oil, a pinch of salt and a few grinds of pepper and spread them on a rimmed baking tray in a single layer. Roasting at this temperature will caramelize their sugars without burning, creating sweet, nutty edges.

Set up a roasting pan with a rack. Place the lamb on the rack (fat side up if applicable). Sear the lamb in a hot oven: roast at 220°C / 430°F for 10–12 minutes to jump-start the Maillard reaction and build a flavorful crust, then reduce the oven temperature to 180°C / 350°F. The initial high heat develops color and flavor; lowering the temperature finishes cooking evenly.

After reducing the heat to 180°C / 350°F, slide the tray of vegetables into a lower rack (not too close to the lamb). Roast for 30–40 minutes more, or until the lamb reaches your desired internal temperature and the vegetables are caramelized and tender. Use an instant-read thermometer inserted into the thickest part of the lamb: 55–57°C (130–135°F) for medium-rare, 60–65°C (140–150°F) for medium. Cooking times vary with oven and bone-in vs boneless — thermometer is the best guide.

When the lamb reaches your target temperature, remove it from the oven and tent it loosely with foil. Let it rest for 15 minutes. Resting allows the meat’s juices to redistribute; if you slice immediately, the juices will run out and the meat will be drier.

While the lamb rests, finish the sweet potato mash: halve or chunk the roasted sweet potatoes and place them in a bowl. Add 2 tbsp unsalted butter and 60 ml warm milk. Mash with a potato masher or use a hand mixer on low speed for a silkier texture. Season with salt and pepper to taste and, if desired, a small pinch of chopped rosemary for continuity of flavor. Warming the milk and butter before adding makes a silkier mash and prevents cooling the potatoes too much.

Slice the rested lamb against the grain into 1 cm thick slices. Arrange on a warmed platter with the caramelized roasted root vegetables and a generous spoonful of sweet potato mash. Spoon any pan juices over the lamb (or quickly deglaze the roasting pan with a splash of red wine or stock on the stovetop and reduce slightly — this creates a quick pan sauce).

Serve immediately, garnishing with a few extra sprigs of rosemary and a drizzle of good olive oil if you like. The contrast of herb-crusted lamb, sweet creamy mash and caramelized roots is the heart of this dish.