Preheat oven to 400 degrees. Spray 9-inch glass pie pan with cooking spray.
Sauté onion in non-stick skillet on medium heat until tender and begins to caramelize.
Add spinach to onion and sauté, stirring until spinach has wilted. Set spinach mixture aside to cool.
In a large mixing bowl, whisk together eggs, baking powder, salt, flour, cayenne pepper and milk. Stir in spinach and onion mixture.
Pour mixture into prepared pie pan and top with feta cheese.
Bake 35 minutes or until center is set and outside edge is golden brown. Let set 5 minutes before serving.