Pre-heat the oven to 180°C (fan)/200°C/gas mark 6.
Meanwhile place a layer of oil in your roasting tin that goes about 5mm up the side of the dish. Place in the oven to allow the oil to get hot.
Remove the roasting tin from the oven and very carefully place the potatoes in the oil (careful it doesn’t spit as the oil will be very hot). Add the rosemary and sea salt and gently mix.