Preheat your oven to 200°C (400°F) or 180°C if you're using a fan setting.
Slice a thin piece off the bottom of each potato so they sit flat, then place one between two wooden spatulas or chopsticks and slice it thinly, about 2mm thick, stopping just before you cut through the bottom.
Rub the potatoes with 1½ teaspoons of olive oil and sprinkle the ¼ teaspoon of salt evenly over all of them.
Set the potatoes on a baking tray and bake for 30 minutes.
Pour the ¼ cup of olive oil over the potatoes, sprinkle the remaining salt on top, and scatter the smashed garlic and rosemary around the tray.
Bake for another 40 minutes, basting the potatoes every 10 minutes with the oil from the tray and gently brushing across the tops to help the slices fan out.
Once the potatoes are tender in the middle, raise the oven temperature to 220°C (425°F) or 200°C fan, baste again, and bake for another 10 to 15 minutes until the edges are deep golden and crispy.
Serve hot and enjoy those crunchy ends first — they’re the best part.