Preheat the oven to 190°C (Gas Mark 5 / 375°F) or in Your Aga, you are looking for the top of the Baking Oven or Grid Shelf on the floor of the roasting oven.
Combine the mascarpone with the diced goat’s cheese and the egg, along with plenty of salt and black pepper. Depending on what goat’s cheese you’re using, the mixture might not blend completely smoothly, but that’s okay. Set aside.
Roll out the puff pastry on a lightly floured surface to around 14 x 10 inches, and transfer it to a baking tray lined with baking paper.
Add the mascarpone mixture, and spread it out with the back of a spoon, only leaving a small gap around the edges of the pastry.
Sprinkle over the thyme leaves - you can add a few whole thyme sprigs for decoration too if you want, though these will probably need to be removed before serving.
Cut each blackberry in half lengthwise, and place them cut-side down onto the tart.
Add another pinch of salt and pepper if desired, and then bake in the oven for 20-25 minutes, or until the pastry is golden brown and puffy, and the cheese mixture has set. Keep an eye on it towards the end of cooking time, as it will turn golden fairly quickly.