Preheat the oven to 240 °C/220 °C fan-forced.

Cut the potatoes into thin discs, about 1.5 mm thick (roughly the thickness of an English penny). If the potatoes are very large, cut each one in half horizontally before slicing.

Slice the whole lemons into discs, the same thickness as the potato slices.

Gently toss the potatoes with the crushed garlic cloves.

Place the whole garlic cloves, half the lemon discs, the rosemary sprig and the combined oils in a roasting pan. Put the pan in the oven to heat up for 5 minutes.

Gently toss the remaining lemon slices with the potatoes and two generous pinches of salt.
Once the oil is sizzling, remove the pan from the oven, discard the whole garlic cloves and rosemary, and add the potatoes and lemons. Toss the potatoes in the oil, return the pan to the oven and roast for 20 minutes. Take the pan from the oven, shuffle the potatoes, lower the heat to 220 °C/200 °C fan-forced and roast for another 15 minutes. After this time, turn the oven function to grill and grill the potatoes for 5 minutes until golden and charred.