Preheat your oven to 375°F (190°C) and prepare a baking dish by greasing it with nonstick cooking spray or a light drizzle of olive oil.

Wash the zucchinis thoroughly under cool water, then slice them into thin rounds, about 1/4 inch thick. In a large mixing bowl, combine the sliced zucchini with fresh spinach leaves.

In a separate medium-sized bowl, crack the eggs and whisk them together until fully combined. Add crumbled feta cheese, garlic powder, salt, pepper, and olive oil to the eggs, then blend until smooth.

Pour the egg and feta mixture over the zucchini and spinach in the mixing bowl. Gently stir the ingredients together until fully coated.

Carefully transfer the combined mixture into the prepared baking dish, spreading it out evenly.

Place the baking dish in the preheated oven and bake the casserole for 30 to 35 minutes, or until the top is golden brown and slightly set.

Once baked, carefully remove the casserole from the oven. Allow it to cool slightly in the dish for about 5 to 10 minutes before slicing.