Add the potatoes to a large pot of well salted water and place over a medium high heat. Cook for 20-30 minutes until a knife slides easily through the biggest potato. Retrieve the potatoes with tongs and allow them to steam dry.
Preheat the oven to 200C or 400F. Once at temperature, pour oil into a large baking tray that is big enough for all your potatoes. Place it in the oven to heat for 3-5 minutes.
Use a large flat bottomed jug (or your hands, depending on how cool the potatoes are) to gently but firmly smash each potato, applying even pressure. Don’t worry if a few bits fall off.
Once the oil is preheated, remove the tray from the oven and work quickly to place all your potatoes in the oil in a single layer. Be careful – if they have any residual water on them the oil will spit. Use a pastry brush to baste the top of each potato with oil so it doesn’t become leathery, and place in the oven for 1 hour.
These potatoes can be cooked up to 1 hour 30 minutes or until the tops are as brown as you’d like them. Just be sure to keep basting them with oil to prevent them drying out. Use a glove or cover your hand with a tea towel as the oven will be hot.