Preheat oven to 350 degrees. Lightly grease 9 inch glass pie pan.
2
Heat butter in large skillet over medium heat. Add onions and cook until translucent, stirring occasionally. Stir in well-drained (squeezed) spinach and continue cooking until moisture has evaporated.
3
In large bowl, combine beaten eggs, milk, cheese, salt and pepper. Add spinach mixture and stir until blended. Pour into prepared pie pan.
4
Bake in preheated oven until center is firm and eggs set, about 45 minutes. Let cool for 10 minutes before serving.