Sift the flour and salt into a large mixing bowl and rub in the Trex with your fingertips until the mixture looks like fine crumbs. Add 3-4 tbsp chilled water and mix it in with a round-bladed knife until the pastry clings together. Form the dough into a ball, then wrap and chill for 10 minutes.
For the filling, heat the Trex in a frying pan and fry the red onions for 3-4 minutes until soft. Add the sugar and cook for 1-2 minutes until starting to caramelise, then add the balsamic vinegar and tomatoes. Remove from the heat and season
Take the tarts from the oven and remove the foil or paper and baking beans. Reduce the oven temperature to 180°C/fan oven 160°C/Gas Mark 4. Share the filling between the tarts and top with the goat’s cheese. Return to the oven for 4-5 minutes to warm through. Serve, garnished with basil leaves.