Preheat oven to 350 degrees.
Heat oil in skillet.
Add onions and mushrooms and saute until they start to brown. (It takes a while. The mushrooms get watery when you start to cook them so cook till moisture has evaporated off and mushrooms just start to brown. We found that white mushrooms are more watery than baby bellas.)
Add spinich, salt and pepper and cook a few minutes more.
Stir in cornmeal.
Lightly grease a 9" pie plate (cooking spray works well).
Press crust mixture into pie plate and shape as a crust.
In separate bowl, beat eggs, sour cream, salt, and pepper.
Fold in cheese and any other filling ingredients, except tomatoes.
Pour filling into crust.
Add a layer of fresh tomato slices on top, if you're doing that.
Bake until done in center, about 40 minutes. (Test with a toothpick.)