Preheat oven to 375 degrees F. Lightly spray a 9x13 inch baking dish with cooking oil spray
In a large bowl, mix flour, salt and baking powder. Stir in eggs, milk and butter. Mix in spinach, onion and mozzarella cheese.
Pour mixture into the prepared baking dish and sprinkle with parmesan cheese. Bake in the preheated oven 30 to 35 minutes, or until a toothpick inserted in the center comes out clean. Cool before cutting into 54 squares.