Thaw and drain the frozen spinach well, squeezing out as much liquid as possible.

Tear the drained spinach into small pieces and place in a large bowl.

In a separate bowl, beat the eggs.

Crumble the feta cheese and add it to the beaten eggs.

Add the egg and feta mixture to the spinach.

Finely chop the onion and mince the garlic clove.

Sauté the onion and garlic in olive oil until softened and lightly browned.

Add the sautéed onion and garlic to the spinach mixture.

Season the spinach mixture with salt, pepper, and oregano to taste.

Preheat oven to 375°F (190°C).

Butter a large oblong baking dish.

Layer half of the filo dough sheets in the prepared baking dish, brushing each sheet with melted butter.

Spread the spinach mixture evenly over the filo dough.

Layer the remaining filo dough sheets on top, brushing each sheet with melted butter.

Bake in the preheated oven for 45 minutes, or until golden brown and crispy.

Let cool slightly before serving.