Bring a medium-sized pot of lightly salted water to a boil.
Cut each potato into chunks (small potatoes in half, medium potatoes into 4 chunks and very large ones into 6). Skip this step if you're using baby potatoes.
Place the potatoes into the boiling water and cook for 5-7 minutes, until they're just tender.
Heat the oven to 200°C, 180°C fan, gas 7, and place a baking tray or a baking dish inside to heat up for a couple of minutes.
Drain the potatoes and transfer them into the hot baking dish. Season with extra-virgin olive oil, garlic, rosemary, sea salt and black pepper, and spread out the potatoes in a nice even layer.
Transfer the potatoes into the oven and bake for 10-15 minutes, until crispy and golden. Halfway through cooking time, remove the pan from the oven, and flip the potatoes on the other side.
Remove the baking tray from the oven, sprinkle the potatoes with extra sea salt and pepper if you like, and let them rest for 5 more minutes before serving.