Preheat oven to 325F (175C). Grease a 9 inch pie pan. In a large skillet, heat oil over medium heat. Add onion and cook, stirring, until soft. Add spinach and cook until any excess moisture evaporates. Meanwhile, combine remaining ingredients in a large bowl. Add the spinach and onion mixture and combine well. Add mixture to pie pan. Bake until for 30 minutes, or until eggs have set. Cool for 10 minutes and serve. Serves 6.