Fry the onion in oil until golden, add the mushrooms and cook until soft.
Add the mushroom cube dissolved in water.
Cook a further few minutes, and add the cornflour paste to thicken, if needed.
(Rivka's comment-- I like the consistency to be fairly thick and creamy, but if you prefer it thinner just add a little water.) To make the cases: Roll our the pastry and cut into circles with a cookie cutter-- you will need to make double the amount of circles than cases needed.
Cut half the circles with a smaller circle cutter to make an O shape in the center.
Brush a beaten egg on to the surface of the solid circles, then lay the O shaped circles on tipe and brush again with the egg.
Bake on greaseproof paper or on oiled baking sheets at 350 degrees F till well risen and golden, about 1/2 an hour.