Preheat oven to 350 degrees F (175 degrees C).

Drain the frozen spinach very well, squeezing out excess moisture.

In a large bowl, combine the drained spinach, grated Mozzarella cheese, halved pepperoni slices, oil, garlic powder, and Parmesan cheese.

In a separate bowl, whisk together the flour, milk, and eggs using an electric mixer until smooth.

Add the flour mixture to the spinach mixture and blend well to combine.

Pour the mixture into a greased 13x9 inch baking pan.

Bake in the preheated oven for 30-45 minutes, or until golden brown and set.

Let cool slightly before cutting into squares.

Serve warm or at room temperature.