Follow these simple steps for perfect Spinach Cheese Squares every time:

1. Preheat Oven & Prepare Pan: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking dish, or line it with parchment paper, leaving an overhang for easy removal. This prevents sticking and ensures clean squares.
2. Prepare the Spinach: Ensure your thawed spinach is squeezed extremely dry. Place the thawed spinach in a clean kitchen towel or several layers of heavy-duty paper towels and wring out as much liquid as possible. Excess moisture is the enemy of good texture here, potentially leading to soggy squares. Set the dry spinach aside.
3. Combine Dry Ingredients: In a large bowl, whisk together the all-purpose flour, baking powder, salt, black pepper, garlic powder (if using), and nutmeg (if using). Whisking ensures the leavening agent and seasonings are evenly distributed.
4. Combine Wet Ingredients: In a separate medium bowl, whisk the lightly beaten eggs. Gradually whisk in the whole milk and the melted butter until well combined.
5. Combine Wet and Dry: Pour the wet ingredients into the bowl with the dry ingredients. Whisk gently until just combined. Be careful not to overmix; a few small lumps are perfectly fine. Overmixing can develop the gluten too much, leading to tough squares.
6. Add Cheese and Spinach: Fold in the thoroughly squeezed spinach, the shredded sharp cheddar cheese, and the grated Parmesan cheese. If using, fold in the chopped green onions or chives now. Stir until everything is evenly incorporated. The batter will be relatively thick.
7. Transfer to Baking Dish: Spread the mixture evenly into the prepared 9×13 inch baking dish. Use a spatula to smooth the top for even baking.
8. Bake: Place the dish in the preheated oven. Bake for 30-40 minutes, or until the squares are set, puffed slightly, golden brown around the edges, and a toothpick inserted into the center comes out clean. The exact time may vary depending on your oven.
9. Cool: Remove the baking dish from the oven and place it on a wire rack. Let the spinach cheese squares cool in the pan for at least 15-20 minutes before cutting. This allows them to set fully, making them much easier to slice cleanly. If you used parchment paper, you can lift the entire block out onto the rack after about 10 minutes of cooling in the pan.
10. Slice and Serve: Once cooled sufficiently, cut into squares of your desired size (e.g., 24 smaller appetizer-sized squares or 12 larger portions). Serve warm or at room temperature.