Place potatoes in a large saucepan. Cover with cold water. Bring to the boil over high heat. Reduce heat and simmer for 10-15 minutes or until slightly softened but still quite firm in the centre. If the surface is still shiny, cook a little longer. Drain and cool slightly.
Scrape the potatoes with a fork to roughen the surface. Place into a roasting pan. Drizzle with oil and toss with the mustard to coat well. Sprinkle with sea salt flakes. Roast, shaking pan occasionally to turn potatoes, for 45 minutes or until golden and crisp.