Preheat your oven to 200°C (400°F), or 180°C (fan-forced). Cut a thin slice off the base of each potato to help them sit flat. Place each potato between two chopsticks or spatulas to prevent cutting all the way through. Slice the potatoes thinly, about 2mm per slice, so you achieve the classic Hasselback look with thin fanned slices. I find that using the chopsticks really helps keep things even and speeds up the process.

Drizzle each sliced potato with 1 1/2 teaspoons of extra virgin olive oil, ensuring it gets into the crevices, and sprinkle over 1/4 teaspoon kosher salt, spreading evenly. Arrange the potatoes on a baking tray, cut side up. Bake in the preheated oven for 30 minutes, allowing the slices to start fanning apart as they cook.

Remove the potatoes from the oven. Pour over 1/4 cup of extra virgin olive oil, making sure to coat each potato, and sprinkle with 1 1/4 teaspoons of kosher salt. Scatter the thinly sliced garlic and rosemary sprigs across the baking tray. Return to the oven and bake for another 40 minutes. Every 10 minutes, use a brush to baste the potatoes generously with the oil from the tray (try to get the oil down between the slices for even crisping). I like to really push the basting brush between the slices for maximum flavor.

Increase the oven temperature to 220°C (425°F), or 200°C fan-forced. Baste the potatoes again with the oil from the tray. Bake for a further 10-15 minutes, until the edges are deeply golden and crispy. Serve immediately while hot, with the crispy edges enjoyed at their best. For a bold flavor, I like to crank up the heat at the end and let the edges get extra crispy—you can always pull them out when they've reached your perfect level of crunch.