1: Firstly, make the pastry. Place the flour, hazelnuts and salt into the bowl of a food processor. Add the cold diced butter and pulse briefly until the mixture resembles the consistency of breadcrumbs. Add the egg yolk and briefly blend again before adding the cold water, a little at a time and mixing until you achieve a soft, smooth dough. Remove from the mixer, wrap in cling film and place in the fridge to rest for at least 1 hour.
2: Pre-heat the oven to 170C / Gas Mark 3.
3: Once the pastry’s chilled, remove from the fridge and roll out on a lightly floured surface, about the thickness of a £1 coin so it’s large enough to line a 10” loose-bottomed tart tin. Line the tin with the pastry, pressing it against the edge of the tin gently and allowing the excess to fall over the sides. Line the inside with baking parchment and fill with ceramic baking beans or uncooked rice / pulses so that it fills the base of the case. Place the case on a baking sheet and bake in the oven for around 15-20 minutes.
4: Remove from the oven, take out the parchment and cooking weights and return to the oven to continue cooking for a further 10-15 minutes, until the pastry case is golden brown, cooked through and nice and crisp. Remove from the oven and allow it to cool. Using a small serrated knife, carefully trim away the excess pastry to tidy the edges.
5: Reduce the oven temperature to 120C / Gas Mark 1.
6: To make the filling, in a large bowl whisk together the double cream, egg yolks, thyme leaves and seasoning. Stir in the grated blue cheese. Pour the mixture into the cooled tart case, still on the baking sheet.
7: Bake in the oven, for 30-40 minutes, until the filling has set and the tart is light golden in colour. Remove from the oven and allow it to cool for 10 minutes in the tin .
8: Carefully remove the tart from the tin, cut into wedges and serve alongside a crunchy salad.