For the pastry:
400 g plain flour
200 g chilled butter, cut into pieces
A pinch of salt
100 g extra mature cheddar, finally grated
8 tbsp chilled water
1 beaten egg, for glazing
For the filling:
350 g butternut squash, peeled, deseeded and cut into small chunks
1 courgette, chopped
2 tbsp olive oil
1 onion, chopped
100 g mushrooms, thinly sliced
300 g jar of roasted red peppers, drained and torn into pieces
100 g pre-packed chestnuts, chopped
75 g extra mature cheddar, grated
1 tbsp chopped fresh thyme
1 egg, beaten
1-2 tsp sesame seeds, for sprinkling
To make the pastry: sift the flour and salt into a large mixing bowl and add the butter. Rub in using your fingertips until the mixture resembles fine breadcrumbs. Stir in the grated cheese, then add 7-8 tbsp of chilled water and stir the mixture with a round-bladed knife so that it comes together to form a soft (but not sticky) dough. Knead lightly for a few moments until smooth, then wrap and chill for 10-15 minutes.
Simmer the butternut squash in lightly salted boiling water until tender-about 10mins. Add the courgette to the sauce pan for the final 2-3mins of cooking time. Drain well.
Meanwhile, heat the olive oil in a frying pan and gently fry the onion and mushrooms until very soft. Remove and set aside to cool. Preheat oven to 180 C/gas 4.
Mix together the squash, courgettes, peppers and chestnuts and season.
Roll out the pastry on a lightly floured surface into a large rectangle and trim the edges neatly to a square measuring about 30 x 30cm.
Spread the mushroom mixture down the centre third of the pastry and pile the vegetable mixture on top. Sprinkle with the grated cheese and herbs. Slash the pastry on each side of the mixture at 2cm intervals, brush with beaten egg, then fold the strips over the filling to overlap like a plait.
Lift the plait onto a lightly greased baking sheet and brush with beaten egg. Sprinkle with sesame seeds. Transfer to the centre of the oven and bake for 25-30 minutes until golden brown. Serve hot or cold with salad.