Heat the butter in a large pan and sauté the onions and garlic together until lightly browned. Add in the mushrooms, season well and cook until softened. Add in the powdered soup, mix together well and cook to thicken then remove from heat. Blend the mixture into a paste and allow to cool down. Lay out the rolled puff pastry and spread over the mushroom mixture evenly. Roll the pastry into a cylinder and using a sharp knife cut 2cm thick slices. Lay onto lined baking sheets, brush the pinwheels with the beaten egg yolk and bake at 220°C for approximately 15 minutes or until nicely browned.