Preheat the oven to 200°C/fan oven 180°C/ Gas Mark 6. Put all the vegetables into a roasting tin, add the melted Trex and toss to mix. Season. Roast for 20 minutes, turning after 10 minutes. Allow to cool.
To make the pastry, sift the flour and salt into a large bowl. Using your fingertips, rub in the Trex until the mixture looks like fine breadcrumbs. Stir in the grated Cheddar, then add 2 tbsp beaten egg and just enough chilled water to make a soft, but not sticky, dough. Knead lightly for a few seconds until smooth, then chill for 10 minutes.
Pile the cooled vegetables onto the pastry, leaving a border of about 10cm (4in) around the edge. Sprinkle the feta cheese and thyme over the top. Fold the pastry edge over the vegetables tucking, overlapping and sealing it to make a free-form tart. Brush with beaten egg.