Preheat oven to 350°F (175 degrees C). Lightly grease a 9 inch pie pan.
Heat oil in a large skillet over medium-high heat. Add onions and cook, stirring occasionally, until onions are soft. Stir in spinach and continue cooking until excess moisture has evaporated.
In a large bowl, combine eggs, cheese, salt and pepper. Add spinach mixture and stir to blend. Scoop into prepared pie pan.
Bake in preheated oven until eggs have set, about 30 minutes. Let cool for 10 minutes before serving.