Preheat the oven to 190ºC.
Peel the potatoes, cut them in half or thirds, then put them in a pan of cold water. Bring to the boil and cook for about 10 minutes. They should not be fully cooked. Drain, then put them back in the pan and shake the pan a few times - this will make them fluffy which will turn into a crunchy texture during roasting.
Put the fat or oil in a large roasting tray. Put the tray in the oven to heat up the fat.
Remove the tray from the oven, then add the potatoes, coating them well in the fat. Be careful, the fat might splash everywhere!
Roast the potatoes for 15 minutes, then turn them over. Roast them again for 15 minutes and turn them over. Then keep turning them every 10 minutes util the potatoes look gold and crispy.
Meanwhile, finely chop the rosemary leaves and mix with the salt and garlic. When the potatoes are almost done, needing only 5 more minutes, add the rosemary & garlic salt. Toss the potatoes, coating each well with the rosemary salt.
Roast for another 5 minutes or so (but not too long, otherwise the garlic and rosemary will taste burnt).
Serve warm.