Soy sauce is added to the mushrooms for a bit of umami flavour. The flavour fades, but deepens the taste of the mushrooms. They’re perfect on a crispy, tender bed of puff pastry.
Heat the butter in a frying pan and add the mushrooms, garlic and thyme. Cook for a few minutes until the mushrooms are soft, darkening and release their juices. Stir in the soy sauce.
Unroll the thawed pastry and cut into six rectangles or squares, and place on baking paper on a baking tray. Score a ½ inch border around each tart. Pile the mushrooms into the centre of each. Brush the edges with the milk and bake for 20 minutes.