1. Preheat oven to 200C. Grease and line a baking tray with baking paper.
2. Make 1cm deep cuts in the skin of the lamb and push a slice of garlic into each. Combine 2 tbsp of the oil, honey, mustard and thyme in a small jug. Season with salt and pepper. Brush honey mixture over the lamb. Place onion in a single layer on the base of a large roasting pan. Top with the lamb. Bake in the oven for 35 miss.
3. Meanwhile, place potatoes in a saucepan of salted boiling water. Cook for 10 miss or until just tender. Drain. Return potatoes to the saucepan and shake to scuff their edges.
4. Arrange the potatoes around the lamb. Drizzle with half the remaining oil and season with salt. Place the pumpkin on the prepared tray. Drizzle with remaining oil. Season with salt and pepper. Roast lamb, potatoes and pumpkin for a further 45 miss for the lamb to be cooked medium-rare. Remove lamb from the oven and rest it 15 mins. Keep potatoes and pumpkin warm.
5. Top lamb with fresh thyme sprigs, slice and serve with roast potatoes, pumpkin, peas and pan juices.