Parboil the potatoes in salted water for 3 minutes. Drain well.
Heat the fat or oil in a large frying pan over a medium-high heat. Add the potatoes, turning to coat, then season with salt, pepper, zest and rosemary. Cook for 5 minutes. Add the lemon segments. Cook, turning, for 10 minutes more until the potatoes are crisp on the outside and fluffy inside.