Prepare Spinach Mixture: Heat a tablespoon of olive oil in a large skillet over medium heat.

Add the finely chopped onion and sauté until translucent.

Add the minced garlic and cook for another minute.

Add the spinach and cook on low heat for 5 minutes, until wilted. Season with a little salt and black pepper. Remove from heat and let cool slightly.

Prepare the Batter: In a large mixing bowl, whisk together the eggs and milk until well combined.

Gradually add the flour and baking powder, whisking until smooth.

Stir in the chopped parsley, and season with a little more salt and black pepper.

Combine and Bake: Preheat the oven to 180°C (356°F).

Gently fold the cooked spinach mixture into the batter.

Stir in the crumbled feta and shredded mozzarella cheese until evenly distributed.

Pour the mixture into a greased baking dish.

Bake: Place the baking dish in the preheated oven and bake for approximately 20 minutes, or until the frittata is set and golden brown on top.

A toothpick inserted into the center should come out clean.

Serve: Let the frittata cool for a few minutes before slicing and serving.

Enjoy warm, with a side salad or crusty bread.