Preheat oven to 220 C / Gas 7. Grease a large rectangle baking tray.
Cut the mushrooms into bite sized pieces. Combine half the butter with the garlic and fry the mushrooms in a shallow pan, adding a little more butter if necessary. Stir in the parsley and remove from the heat.
Roll pastry into a rectangle approximately 20x12cm (8x5 in). Lift carefully onto prepared baking tray. With a sharp knife score a small rectangle 4cm (1 1/2 in) from the edge, cutting only half way through the pastry.
Scoop the mushrooms from the pan and place in the inner rectangle, trying not to let them hang over into the outer rim.
Bake for 15 minutes until the pastry has risen. Dot with the remaining butter, then close the oven for another 5 to 7 minutes until pastry is risen and golden and mushrooms are sizzling. Serve immediately.
This recipe works well with dark mushrooms like chestnut or mini portabella rather than white button mushrooms.