One rectangular sheet pre-rolled all butter puff pastry,
1/2 red onion
400g Charlotte potatoes
2 tbsp vegetable oil
Sea salt flakes
120g gorgonzola piccante
3 tbsp double cream
1 egg yolk
1 tbsp milk
Black pepper, freshly ground
Few sprigs of rosemary
Preheat the oven to 220C / 200C fan.
Line a baking sheet with greaseproof paper and lay out the pastry rectangle on top. Using a small sharp knife, score the pastry along each edge - 2.5 centimetres in from the edge - to create a crust. Prick the base of the pastry, inside the crust, all over with a fork then put it into the fridge while you prepare the toppings.
Finely slice the onion and put it into a bowl with 1 teaspoon of olive oil and a pinch of salt. Toss it around to cover the slices well.
Peel the potatoes and slice them on the fine setting of a mandolin, or very carefully with a knife as thin as you can. Put them into another bowl with the remaining oil and toss that well, with a few large pinches of sea salt.
Put the mascarpone and cream into a bowl and mix together well. Crumble in the gorgonzola and mix it so that it’s half combined, but there are still a few lumps of it remaining. Bring the pastry out of the fridge and spread the cheese filling evenly all over it, inside the crust lines. Scatter the red onion slices on top, then arrange the potato slices in neat rows, so that they slightly overlap. Mix together the egg yolk and milk, and brush it over the crust. Grind black pepper on top then put the tart into the hot oven and cook for 25-30 minutes, until the crust is risen, and the potatoes are browning and starting to curl up. Keep an eye on the pastry: if it starts to darken more than you like, cover the crust with foil while you carry on browning the potatoes. Once cooked, take out of the oven and transfer to a wire rack to cool for 5 minutes, then serve.