Preheat your oven to 350°F (175°C).
- Grease a 9×13-inch baking dish or line it with parchment paper.
- In a large bowl, beat the eggs, then stir in the milk and olive oil or melted butter.
- Add the flour and baking powder, mixing until just combined.
- Stir in the chopped onion, drained spinach, cheddar cheese, Parmesan cheese, salt, and pepper.
- Pour the mixture into the prepared baking dish and spread evenly.
- Bake for 30–35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
- Let cool slightly before cutting into squares and serving.