Preheat Oven: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish and set aside.
Cook the Spinach: Place the spinach in a medium saucepan and add enough water to cover. Bring to a boil, then reduce the heat to a simmer and cook for about 3 minutes until the spinach is limp. Drain the spinach well and set it aside.
Prepare the Batter: In a large mixing bowl, whisk together the flour, salt, and baking powder. Stir in the eggs, milk, and melted butter until well combined. Add the cooked spinach, chopped onion, and shredded mozzarella cheese to the batter, stirring until everything is evenly distributed.
Bake: Pour the mixture into the prepared baking dish, spreading it out evenly. Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve: Allow the spinach brownies to cool in the baking dish before cutting them into squares. Serve warm or at room temperature for a delicious and savory snack!