Preheat the oven to 200°C/fan 180°C/gas 6 (see tip). Put the new potatoes in a pan of cold salted water and bring to the boil. Reduce the heat, then simmer for 10 minutes until just tender. Drain well and tumble into a roasting tray.
Press gently with a fork to crush slightly, then toss with the oil and roast for 20-25 minutes until golden and crispy, shaking gently halfway through cooking. Melt the unsalted butter in a small pan over a low heat. Let the butter bubble until it starts to turn brown and smell nutty, then immediately drizzle over the potatoes. Season with sea salt and serve with the lamb.