Lightly dust the workbench with some flour, and roll out the puff pastry to a thickness of 3-4mm. Cut out a pastry circle 2cm wider than your frying pan (ideally a cast-iron skillet, 22-25cm in diameter), and place on a tray lined with baking paper. (If you are using ready-rolled puff pastry, fold it in half and re-roll so it is large enough to cut out the circle.) Pierce the pastry all over with a fork, then place in the fridge to chill.

Cut the pumpkin into large chunks (about 4 centimetres). Steam, or microwave on full power, for about 10 minutes, until just soft enough for a small paring knife to penetrate with some resistance, then set aside.

Combine the lime zest, chopped pistachio, the 1 tablespoon of extra sugar and ginger powder in a small bowl and set aside.

Preheat the oven to 210°C (190°C fan-forced). 

 Arrange the butter slices to cover the entire base of the skillet in one layer. Rub the vanilla seeds into the 120 grams of caster sugar, then sprinkle evenly all over the butter, covering it entirely. Arrange the hot pumpkin pieces on top of the vanilla sugar, packing them in quite tightly so there are no large gaps. Place the skillet on a medium heat, and allow the sugar to amalgamate with the butter – it will start to brown at the edges first, then begin to bubble and, after about 10 minutes, form a golden caramel all over. Resist the temptation to stir, but gently swirl the pan on the hob to promote even browning.

Once the caramel is bubbling and golden brown all over, remove from heat and scatter all but 1 tablespoon of the pistachiolime mix over the pumpkin. Drape the cold puff pastry over the pumpkin and, working quite quickly, use a butter knife to tuck in the edges of the pastry so that it will form a thin lip around the pumpkin when inverted. Place onto the middle shelf of the oven and bake for about 25 minutes until the pastry is light golden brown and cooked.

Meanwhile, have a large plate (larger than the skillet, and with a rim to catch the liquid caramel) close at hand. Remove the tart from the oven, carefully place the serving plate on top and quickly flip the tart out onto the plate. Lift the skillet away and use a small paring knife to carefully dislodge any pumpkin pieces which have stuck to the skillet, and place back onto the tart. Sprinkle the remaining pistachio and lime mix over the top and serve with crème fraîche.