Coat a 9-inch pie plate with cooking spray; set aside.
Combine flour, salt, oregano, and baking powder in a small bowl; set aside.
Beat eggs with a whisk until frothy (about 1 minute); add milk, cheeses, onion, and garlic. Add spinach and olives, stirring mixture well with a fork. Using a rubber spatula, fold the reserved dry ingredients into the spinach mixture. Transfer to prepared pie dish, and smooth top.
Bake at 350° for 45 minutes or until the filling is slightly puffed, lightly browned around the edges, and set in the center. Remove from oven, and cool 5 minutes before serving.