To make the lemon salt, crush and mix together the lemon zest and salt in a mortar and pestle until the salt is fragrant, colored, and ready. Place in a dish. Use the required amount immediately or let it dry for a few hours before storing. If it hardens, simply crush it a bit before storing in a jar.

Heat 6-8cm / 2-3 inches of sunflower oil in a saucepan and bring to frying temperature. You can do this by using a thermometer or by placing a small piece of potato in the cold oil before heating it. When the potato floats and turns a deep golden brown, the temperature will have reached 180°C / 350°F / gas 4, and it will be ready to start frying (don’t forget to remove the potato piece before starting).

Pat the strips with paper towels to remove excess starch. Make sure you have a slotted spoon or skimmer (like a flat colander with a handle) and a large pile of paper towels on the side. Carefully place a portion of the potatoes in the oil (don’t overload) for a few minutes until golden brown and crispy. Cook all the potatoes in batches like this. Add the rosemary for the last 30 seconds. Remove the fries and rosemary to place on the paper towels to absorb excess oil and sprinkle with lemon salt. Serve immediately.