Preheat oven to 190C/375F/Gas mark 5, or for convection cooking: 170C/350F.
Peel potatoes and cut in half. Put in a pan of salted cold water. Bring to the boil and simmer for 7-8 minutes until almost cooked (they will still feel firm when pierced with a knife). Drain thoroughly using a colander, then shake the potatoes around a little in the colander until the outsides are roughened.
Place in roasting pan, sprinkle with avocado oil, rosemary. Season with salt and pepper. Cook in oven at 190C/375F until golden crisp, about 45-60 minutes.