Parboil the onions whilst making the pastry. Line an 8-in. fireproof plate with half the pastry. Mix the salt and pepper with the flour. Slice the onions and dip in the seasoned flour, spread them Over the bottom of the lined plate. Grate the cheese and sprinkle it over the onion, add the milk. Wet the edge of the pastry, put on the cover and press the edges firmly together. Knock up the edges, decorate as desired and brush over with milk. Bake in a hot oven (425° F., Gas 7) for about 40 min.