Hand mix all dough ingredients until they come out of your hands without sticking. Stretch dough to fit evenly on bottom and upsides of a pie crust pan (ring mold pan optional). There is no need for greasing. Gently poke bottom of dough with a fork several times. Reserve.
Rinse mushrooms and drain well. Slice and fry mushrooms together with onions in hot oil. Season with salt.
In a bowl mix to a cream the egg, milk, wheat flour, salt, pepper and oregano. Reserve this cream.
Spread mushrooms over the dough and bake in a pre-heated oven (200 C/375 F/British Gas Mark 5) for 10 minutes. Remove from oven, pour in the cream, sprinkle with cheese, garnish with tomato slices and bake for an extra 20 minutes.