To make the pastry
Put the flour, 50g hazelnuts and ½ teasppon of sea salt into a food processor. Give it a quick pulse to disperse the nuts through the flour, then add the butter and blitz until the mixture resembles rough breadcrumbs.
Add the egg and a couple of teaspoons of water, and blitz again until the pastry is just coming together - if the pastry feels dry, add another teaspoon of water and pulse again.
Chill the pastry in the fridge for 30 mins, until firm.
Roll the pastry into a circle roughly 3mm thick on a floured surface, then use it to line a deep 25cm tart tin. Prick the base all over with a fork, then return to the fridge for 20 mins to firm up, or chill it in the freezer for a few minutes.
To make the filling
Heat oven to 180C /Gas 6.
Place the squash chunks into a roasting tin, drizzle with a little oil, add a few knobs of butter to the pan and season with sea salt. Roast for 35 minutes, or until tender but holding their shape. Set aside to cool.
In a pan add a little butter and sweat the onions for around 7-8 minutes until they are translucent. Then set aside.
Whisk the cream with the milk and eggs in a bowl until combined, add a good grating of nutmeg, season well, then stir through the chicken, cooled squash, onions, thyme and parsley and half the cheddar.
To make the tart
Line the pastry with baking parchment, then fill it with baking beans or dried rice and bake for 15 minutes until the sides of the pastry are holding their shape. Remove the paper and beans and bake for 10 minutes more until the pastry is lightly golden. Reduce the oven to 160C/Gas 3.
Pour the quiche filling into the pastry case, then scatter over the rest of the hazelnuts and cheddar. Return to the oven for 40-50 minutess or until the filling is just set.
Leave the tart to cool and serve at room temperature.