Preheat the oven to 200°C fan forced.
Heat the oil in a medium (about 22cm base) ovenproof frying pan over medium heat. Add the leek and garlic and cook 3 minutes until soft. Add the thyme. Add the mushrooms, stalk side down. Increase heat to medium-high and cook 5 minutes.
Meanwhile, cut a 23cm round from each sheet of pastry. Place the rounds on top of each other and press together. Spread half the cheese over one side of the pastry round.
Remove the frying pan from the heat. Place the pastry round cheese side down over the mushrooms, tucking the pastry under the mushrooms. Transfer the frying pan to the oven and bake for 20-25 minutes or until pastry is golden and puffed. Place a board or wire rack on top of the pan and invert. Crumble over the remaining goats cheese, top with rocket, season with pepper and serve.