1-First Step: Preheat and prepare the baking dish Start by preheating your oven to 350°F. Place the 2 tablespoons of melted butter into a 13 x 9-inch baking dish and spread it around lightly if needed. This step gives the bottom a richer flavor and helps keep the squares from sticking. Using the correct dish size is important here. A 13 x 9-inch dish gives the batter enough room to bake evenly and makes it easier to cut the finished spinach cheddar squares into neat portions later.

2-Second Step: Mix the wet ingredients In a large bowl, beat the 3 eggs until they are well blended. Stir in the 1 cup of milk. This mixture forms the base of the recipe and helps bind the filling together. If you are cooking for kids or picky eaters, this is a good point to check the mixture texture. It should look smooth before you add the dry ingredients. That helps the final squares bake with a light, even crumb.

3-Third Step: Add the flour and baking powder Add the 1 cup all-purpose flour and 1 teaspoon baking powder to the bowl. Mix until everything is well blended and no dry patches remain. The batter will be thick, and that is normal for this style of spinach and cheese squares. Do not overmix once the flour goes in. Stir just until combined so the finished bake stays tender. If you want a slightly lighter texture, whisk the dry ingredients together before adding them to the eggs and milk.

4-Fourth Step: Stir in the cheese, spinach, and onion Add the 4 cups shredded Cheddar cheese, the 2 packages of thawed and squeezed chopped spinach, and the 1 tablespoon finely chopped onion. Stir until the ingredients are spread evenly through the batter. The mixture should look thick, colorful, and fully combined. This is where the recipe gets its strong savory flavor. The Cheddar adds richness, the spinach gives body and color, and the onion adds a mild sharp note. If you want more background on spinach nutrition, you can read this helpful guide from Cleveland Clinic on the benefits of spinach.

5-Fifth Step: Transfer to the dish and bake Pour the mixture into the prepared baking dish. Spread it out evenly with a spatula so the top bakes at the same rate across the whole pan. Place it in the preheated 350°F oven and bake for 35 minutes, or until the center is set. The top should look lightly golden and the middle should no longer jiggle when you tap the pan gently. If your oven runs hot, start checking a few minutes early. If it runs cool, you may need a little extra time. The goal is a firm center that slices cleanly.

6-Final Step: Cool, cut, and serve Once baked, allow the spinach and cheese squares to cool for 30 minutes. This rest time helps the squares firm up and makes cutting much easier. After cooling, cut the pan into 6 rows by 4 rows to make 24 squares. Serve them immediately for the best texture. They are especially nice when warm, but they also hold together well enough to serve at room temperature for brunch or a buffet table. If you enjoy savory baked dishes, you may also like the crab stuffed cheddar bay biscuits recipe for another crowd-friendly option.