Place potatoes in a large pot, cover with 10cm of water and add salt. Place over high heat and bring to a boil. Partially cover with a lid and boil for 25 minutes or until they offer no resistance when a knife is inserted into the centre. Don't worry if the skin splits.
Halfway through boiling, preheat oven to 180C fan-forced (200C conventional).
Drain potatoes then transfer to a baking tray. Using a "S" curve potato masher or two forks, smash the potatoes so they're 1½–2cm thick. If the potatoes split, don't worry, just push them together. Aim for raggedy surfaces – it means more crispy bits.
Leave potatoes on the tray for 10 minutes to steam dry – this makes them crispier. Then drizzle over butter and oil, and sprinkle with salt and pepper.
Roast for 40-55 minutes or until deeply golden and crispy on the top and underside. Do not flip!
Remove from oven, sprinkle with parsley or thyme and serve immediately.
