Method
1. Peel the potatoes and cut into 3mm-thick slices. Cook in a pan of boiling water for 4 minutes or until just tender but still holding their shape. Drain carefully so they don’t break up, then set aside to cool and dry on a clean tea towel.
2. Meanwhile, preheat the oven to 200°C/fan180°C/gas 6. Roll out the pastry on a lightly floured surface to a thin, about 36cm square. Using about a 16cm plate or bowl as a template, cut out 4 pastry circles and pop onto 2 large, non-stick baking sheets. Discard the pastry trimmings.
3. Arrange half the potatoes on the pastry circles, leaving a small border all around. Spread the potatoes with half the cream, season and crumble over half the cheese. Top with nearly all the onions (save the rest for garnish) and the remaining potatoes. Sprinkle with the remaining cheese, then place a small dollop of the remaining cream in the centre of each. Season with black pepper only.
4. Bake the tarts for 25-30 minutes, swapping the trays over in the oven halfway, until the pastry is cooked and golden and the topping is melting. Cool on the trays for about 5 minutes, then transfer each to a serving plate. Top with the reserved spring onions to garnish. Serve with a mixed green salad.