Heat oven to 200C/180C fan/gas 6.
Toss the squash in a roasting tin with the
garlic, rosemary and thyme leaves. Drizzle
with 2 tbsp oil and season well. Roast for
40 mins or until soft and starting to brown.
Remove and squeeze the garlic out of
its skin, mash and gently mix with the
squash, than set aside to cool.
Heat remaining oil in a small pan and
fry the onion until soft and lightly golden.
Add the mushroom and fry for 3-4 mins
more. Remove from heat and set aside.
In a large bowl, mix the chestnuts, herbs,
cheese, and nutmeg. Stir in mushrooms
and squash mixture, taking care not to
break up the squash too much. Season well.
Roll a third of the pastry into a rectangle
measuring 12 x 30cm. Place on a baking
tray and pierce all over with a fork, brush
with beaten egg and bake for 10 mins
until lightly golden and slightly risen.
Roll out the remaining two-thirds to
a rectangle slightly larger than the base
then, using a sharp knife, slash diagonally.
Pile the filling onto the partly-cooked
base leaving a 1cm border. Brush the
border with a little beaten egg and
carefully lay the uncooked pastry on top,
pressing down the edges to seal. Glaze
top with a little more egg and bake for
30-35 mins until the pastry is golden,
risen and cooked through.