For the pastry
Preheat the oven to 200°C, 400°F, Gas 6 and lightly grease a baking sheet.
Put the Trex into a roasting tin and melt in the oven for 2-3 minutes. Remove from the oven and add the butternut squash, pepper, onion and cumin seeds then return to the oven and roast for about 30 minutes, until tender. Add the tomatoes and set aside to cool, leaving the oven on.
Meanwhile, make the pastry. Sift the flour and salt into a bowl and rub in the Trex using your fingertips until the mix resembles fine crumbs. Stir in the cheese then add 7-8 tbsp of chilled water. Bring the mix together to form a soft, not sticky, dough. Knead until smooth then wrap in clingfilm and chill for 10-15 minutes.
Roll out the pastry onto a lightly floured surface to 35 x 30cm/14 x 12in and trim the edges neatly. Pile the cooled vegetables down the centre then sprinkle with the cheese and thyme.
Slash the pastry on each side at 2cm/¾in intervals, brush with beaten egg then fold over the filling to overlap like a plait.
Lift onto the prepared baking sheet and brush with beaten egg. Sprinkle with sesame seeds, transfer to the oven and bake for 25-30 minutes until golden brown.