Preheat oven to 200 C (390 F). Line a 15 x 22 cm baking tin with one piece of baking paper so that the base and the two longest sides are covered, and baking paper is hanging over the edges. This will make it easier to lift out once cooked.
Mix spinach, zucchini, half of the cheese (so 1/4 cup), and eggs together.
Add flour and mix until well combined.
Pour into prepared baking tin and sprinkle the remaining 1/4 cup of cheese evenly on top.
Bake for approximately 22 minutes, until top is starting to brown and it feels firm.
Remove from oven and let cool for about 10 minutes.
Use the overhanging baking paper to easily lift the whole thing out of the baking tin. You may need to run a knife along the short ends if it has stuck a little where there is no baking paper.
Leave to cool completely before cutting into bars.
Store these in an airtight container in the fridge for up to three days.