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Heat oven to 200C/180C fan/gas 6. Put the hazelnuts in a small roasting tin and roast for a few mins until they start to deepen in colour and smell nutty. Tip into a bowl and leave to cool completely, then chill until needed. You could do this step the night before to ensure they’re nice and cold.
Use the same roasting tin to roast the broccoli and cauliflower (without the leaves), tossed in a little oil for 10 mins until starting to soften and char in places. To make the sauce, melt the butter in a pan over a medium heat, then add the flour, crème fraîche, cheese, mustard and some black pepper. Set back over the heat, whisking continuously until the sauce is smooth and thick and the cheese has melted. Cover the surface with cling film (to prevent a skin from forming) and set aside to cool.
Tip most of the nuts into a food processor, reserving 8-10 for the top. Pulse briefly until finely chopped. Add the flours and ½ tsp salt, and whizz again until combined. Add the butter and continue pulsing until the mixture resembles breadcrumbs, then add the egg and 2-3 tbsp cold water and whizz until the mixture starts to form bigger clumps. Tip onto a work surface and knead briefly to bring the pastry together. Shape into a disc, wrap in cling film and chill for at least 20 mins. Heat oven to 200C/180C fan/gas 6 (if not already on) and line a large baking sheet with parchment.
When the cheese mixture has cooled, mix in the whole egg. On a lightly floured surface, roll out the pastry to a circle about the thickness of a £1 coin (don’t worry if the edges look rustic, this is all a part of its charm). Transfer it to the baking sheet. Spread the cheese mixture over the pastry, leaving a border of about 2cm around the edge. Arrange the veg and leaves on top of the cheese sauce. Roughly chop and scatter over the remaining hazelnuts, then fold the sides of the pastry in to create a nice edge around the tart. Brush with the egg yolk and bake for 45 mins until the pastry is golden and crisp and the filling is bubbling. Eat warm.
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