Preheat oven to 350 degrees. In large skillet, sauté onion in oil until tender. Add spinach and cook until excess moisture evaporates; set aside to cool. In a large bowl, combine eggs, cheese, salt and pepper; stir in spinach mixture. Pour into a greased 9 inch pie plate. Bake for 30 minutes, or until set. 6 servings.
*If using frozen bagged spinach, partially thaw before adding to sauteed onion.