Notes: You can thaw the spinach in the microwave or overnight in the refrigerator. Cooled Spinach Squares can be refrigerated for up to 24 hours and reheated, covered in foil, in a 375 degree F oven. Makes 32 squares
1. Adjust oven rack to upper-middle position and heat oven to 375 degrees F. Spray 13-inch by 9-inch baking dish with vegetable oil spray. Whisk flour, baking powder, salt, pepper, and cayenne together in bowl. Add broth and eggs and whisk until smooth. Stir in spinach, Gruyere, onion, and garlic until combined.
2. Transfer mixture to prepared baking dish and sprinkle with Parmesan. Bake until browned on top and bubbling around edges, 40 to 45 minutes. Let cool in pan for 20 minutes. Cut into 32 equal-size squares. Serve warm.
Test Kitchen Technique: Squeeze Spinach Dry
To prevent soggy spinach squares, squeeze the spinach dry before you stir it into the batter. Place the spinach on a clean dish towel, gather the edges, and wring dry.