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First off, make the pastry. Put the pumpkin seeds in the bowl of a food processor and pulse briefly to a crumb. Add the flour, thyme, parmesan (or vegetarian alternative) and ½ tsp salt and pulse again to combine. Add the butter and pulse to a breadcrumb consistency, then add the egg yolk and blitz again. Add iced water to the mix, 1 tbsp at a time, blitzing between additions until the dough clumps together (ours took 2 tbsp to reach this consistency). Tip the dough out into a bowl and bring together into a smooth ball with your hands, adding a little more flour if it’s sticky. Wrap in baking parchment and chill for 30 mins.
Heat oven to 200C/180C fan/gas 6. Once rested, dust a work surface with flour and roll the pastry out to the thickness of a £1 coin. If it’s difficult to work with, you can do this between two pieces of baking parchment. Line a 23cm round tart tin with the pastry, prick the base, then fill with parchment and baking beans. Blind bake for 15 mins, then remove the beans and parchment and cook for a further 12 mins. Remove from the oven and reduce the temperature to 180C/160C fan/gas 4.
Heat the olive oil and butter over a medium-high heat. Gently fry the leek with a pinch of salt and a grinding of fresh black pepper for 6-8 mins until softened and aromatic. Add the apple and brussels sprouts and fry for a further 5 mins until the sprouts are softened and the apple is colouring. Remove from the heat and squeeze over the lemon juice, tossing to coat.
Combine the double cream, yogurt, cheese, eggs and a good grating of nutmeg. Season and whisk until combined. Pour half the custard into the tart case and top with half the leek and sprout mix. Pour in the rest of the custard and top with the remaining sprout mixture. Dot the walnuts around the top of the tart.
Bake for 20-25 mins until puffed and golden, then remove from the oven and allow to settle for 10 mins before slicing. Scatter over a little of the grated comté (or vegetarian alternative) to serve.