Heat oil in skillet, and sauté onions for 4 minutes, or until they become tender. Toss in the mushrooms and keep sauteing for 5 minutes more. Set aside.
Beat eggs in a large bowl.
In another bowl, dissolve cornstarch in milk and beat until this texture is smooth, then add eggs.
Dissolve chicken soup mix in hot water and mix this thoroughly, then add to egg mixture.
Put the rest of the ingredients into mixture including the mushrooms and onions, and blend well.
Pour into a greased pie plate, and bake at 350F for 35-45 minutes or until lightly browned.