FOR THE PARMESAN & WALNUT PASTRY
200g plain flour, sifted
100g finely ground walnuts
20g finely grated Parmesan
100g cold unsalted butter, diced
1 medium egg, lightly beaten
1-2 tbsp whole milk, to bind
FOR THE MUSHROOM FILLING
25g unsalted butter
2 shallots, finely chopped
leaves from a few sprigs of thyme
350g mixed fresh wild mushrooms, large ones sliced
200ml crème fraîche
150ml double cream
1 large egg
1 large egg yolk
½ tsp fine sea salt
½ tsp freshly ground black pepper
Put the flour, walnuts, cheese and a pinch of fine sea salt into a food processor and pulse for a few seconds to mix. Add the cold butter and whiz into fine crumbs. Tip in the egg and pulse until the mixture comes together, adding a little milk, as necessary, to form a dough. Tip the dough onto a lightly floured surface and knead gently. Wrap in cling film and chill for at least 30 minutes before using.
Roll out the pastry on a lightly floured surface to about the thickness of a £1 coin, then use it to line a 23cm tart tin, 3-4cm deep, with a removable base. Press the pastry into the edges of the tin and leave a little excess overhanging the sides. Chill for at least 30 minutes.
Preheat the oven to 190C/fan 170C/gas 5. Line the pastry with foil and fill with baking beans. Bake blind for 15-20 minutes until the sides are lightly golden. Remove the foil and beans and return to the oven for another 5 minutes until the base is golden and there are no uncooked patches left. Remove from the oven and reduce the oven temperature to 170C/ fan 150C/gas 3. Use a sharp knife to trim off the overhanging pastry, then set aside to cool.
Meanwhile, prepare the filling. Melt the butter in a wide frying pan and add the shallots. Sauté the shallots for 3-4 minutes until they begin to soften. Add the thyme leaves, mushrooms and some salt and pepper. Fry the mushrooms over high heat for 4-5 minutes until any moisture they release has been cooked off. Transfer to a bowl and leave to cool.
For the custard, whisk together the crème fraîche, double cream, egg and egg yolk, salt and pepper in a medium bowl. Once cooled, spread the mushrooms evenly in the tart shell and pour over the custard. Bake the tart for 35-45 minutes until the custard is golden and slightly puffed. Remove from the oven and leave to cool for a few minutes before unmoulding. Slice and serve warm or at room temperature.