Preheat the oven to 200°C, 400°F, Gas 6. Heat the butter in a frying pan and sweat the shallots without colouring for 3-4 minutes. Add the mushrooms and garlic, thyme and a little salt and cook until the mushrooms release their juices and they evaporate. Season and set aside to cool. Roll out the puff pastry on a lightly floured surface to about 2cm thickness. Cut out circles in the pastry using a saucer, two per person. Score a narrow border around one of the circle pairs, then place the pastry circles on nonstick baking paper on a baking sheet and bake in the oven for 7-10 minutes or until puffed and golden. Hollow out the middle of the scored circles with a spoon to leave a cavity. Fill the cavity with the mushroom mixture and some cheese, then put the lid halves on top. Return to the oven and bake for another 10-15 minutes until puffed and golden and the cheese is bubbling.
