Preheat oven to to 180. Oil deep round cake pan. Heat oil in frying pan and cook onion until soft, add mushrooms and cook til tender. Add spinach til it wilts. Drain off any liquid. Whisk eggs and whites together, with milk and cheese. Stir in vegies. Pour into pan and bake in oven for 30 mins or til set. Can be browned under a grill.