Method

1. Preheat the oven to 200°C and line a baking tray with baking paper.
2. Peel the potatoes and slice thinly using a mandolin (approximately 3-4mm thick).
3. Place the potato slices into a saucepan, cover with water and bring to a gentle boil. Par-cook for about 7 minutes, until just tender but still holding their shape. Avoid overcooking, as the slices may break apart.
4. Drain the potatoes and rinse under cold water to stop the cooking process. Set aside to dry slightly.
5. In a small bowl, mix the grated Parmigiano Reggiano with hot water to form a spreadable paste.
6. Lay the puff pastry onto the lined tray. Using a knife or pizza cutter, lightly score a border approximately 1cm from the edge, being careful not to cut all the way through. Prick the centre area (inside the border) with a fork to prevent it from puffing.
7. Spread the cheese paste evenly over the centre of the pastry, avoiding the border. Arrange the potato slices over the top, slightly overlapping, then scatter generously with fresh rosemary.
8. Brush the pastry border with milk. Drizzle the tart lightly with olive oil and sprinkle with sea salt.
9. Bake for approximately 20 minutes, or until the pastry is puffed and golden and the edges are crisp.
10. Remove from the oven and serve whilst crispy and warm.