Preheat the oven to 200°C, fan 180°C, gas 6. Put the potatoes in a large pan of cold salted water, cover and bring to the boil. Simmer for 6-8 minutes, drain and return to the pan, shaking to rough up the edges.
Put the fat in a large roasting tray and pop it in the oven for 5 minutes until smoking hot. Carefully add the potatoes, season and toss to coat. Roast in the oven for 30 minutes.
Add the shallots to the potatoes, toss well and return to the oven for a further 25 minutes. Sprinkle over the rosemary and roast for another 5 minutes, or until the potatoes are golden. Serve.