Preheat the oven to 200°C, fan 180°C, gas 6. Using a pestle and mortar, grind the salt, lemon zest and tablespoon of thyme leaves together. Set aside.
Place the potatoes in a large saucepan and fill with enough cold water to cover. Bring to the boil, then reduce the heat and simmer for 5 minutes. Drain the potatoes and return to the pan. Shake the pan to roughen the edges of the potatoes and allow to dry for 2 minutes.
Coat the potatoes with melted butter and spread in a roasting tin with the thyme sprigs. Roast for 20 minutes, turn the potatoes and continue to cook for a further 15 minutes until golden and crispy.
Remove the roasting tin from the oven and sprinkle the potatoes with the lemon thyme salt. Serve any leftover salt mix on the side.
Cook’s tip: Short on time? There are lots of nutrients in potato skins, so leave them on.