- Preheat oven 350 degrees.
- Heat the half the butter or margarine and the corn oil in a large skillet. It needs to be super hot. Add the half the mushrooms and half the onion. Sauté until browned. If your mushrooms are wet, even with the very high temperature, they may release liquid. If this happens, continue to cook until the liquid evaporated and mushrooms brown a little.
- Cook the second half of mushrooms and onions in the other half of butter and oil. Beat the egg slightly and add some of the hot mixture to it. Add the egg mixture to the skillet along with the cheese, thyme, and the parsley. Cook about one minute longer and set aside.
- Fill each tart shell with one to two tablespoons of the mushroom filling. Top with a sprinkling of parmesan cheese. Bake at 350 degrees for 20 to 25 minutes or until cheese is melted and tarts are lightly browned.