Beat eggs and milk thoroughly. Add dashes of salt and pepper, then mushrooms. Heat oil in large skillet and fry the onions until soft and golden. Tip the eggs mixture into the pan. Fry on one side 1 - 2 minutes, until gold and fairly firm. Slide omelet out onto a large plate and flip back into pan on uncooked side. Cook until done. Serve warm, or allow to cool then cut into wedges to accompany a fresh salad.