Heat the oven to Gas 5,190°C, 375°F. Peel the potatoes, cut into even sizes and parboil for 9 minutes.
Heat the fat or oil in a large roasting pan. Drain the potatoes well and toss to scuff up the outside (or scratch with a fork) to add crispiness. Place in the fat or oil and coat well all over. Roast for 25 minutes. Check and turn the potatoes if needed, then return to oven for a further 25 minutes.
Chop the rosemary finely. Take the potatoes out of the dish, put on kitchen paper to absorb excess oil, then serve scattered with chopped rosemary.
Tana's book Home Made: Good, Honest Food Made Easy (Harper Collins, £20) is a great recipe compilation based around key ingredients used in home cooking.