Bring a large pan of salted water to the boil, then add the potatoes and cook for 8 minutes to par-boil them. Drain, then let the potatoes sit in a colander or sieve over the saucepan for a while until they have completely dried and are fairly cool. If you don't have time to wait, tip them into a clean tea towel to soak up any excess moisture - this will guarantee a really crispy skin.
Preheat the oven to 180°C / 350°F / Gas Mark 4. Put the olive oil into a roasting tin along with the garlic and rosemary stalks, and place in the oven whilst it is heating up. This will heat the oil and help the garlic and rosemary to infuse their flavours into the oil.
When the oven is hot enough, remove the roasting tin and carefully place the potatoes into it. Season generously with salt and black pepper, then stir well until all the potatoes are fully coated in hot oil.
Roast the potatoes for 35-45 minutes, turning occasionally, until they are golden brown and crispy, and cooked through to the centre, (test some of the largest ones with a sharp knife). Discard the garlic cloves and rosemary, then serve.