Preheat the oven to 190C/170C Fan/Gas 5. Boil the potatoes in lightly salted water until cooked through. Drain and leave to cool, then slice them thinly.
Heat half the oil in a large frying pan and sauté the potatoes for 5–10 minutes, or until golden-brown on both sides.
Meanwhile, heat the remaining oil in a large, ovenproof frying pan over a high heat. Sear the lamb for 1 minute on each side, until the fat has rendered. Transfer to the oven for 5 minutes, then leave to rest for 3 minutes.
Meanwhile, boil the broccoli florets in salted water for 4 minutes. Drain and blend with a hand-held blender until smooth. Check the seasoning.
Boil the asparagus, purple and white sprouting broccoli in salted water for 2–3 minutes. Drain.
Heat the butter in a large frying pan. When foaming, add the garlic and rosemary and cook for 1 minute. Pour this over the lamb and carve into thick slices.
To serve, place the broccoli purée in the centre of a serving dish. Scatter over the new potatoes and arrange the lamb slices around them. Nestle in the broccoli and asparagus spears and drizzle with rapeseed oil.