Preheat your oven to 375°F (190°C) and grease a medium-sized casserole dish with olive oil or cooking spray.

In a large skillet over medium heat, add olive oil and sauté the chopped onion until it becomes translucent, about 4 minutes.

Stir in the minced garlic, cooking for an additional minute until fragrant but not browned.

Add the chopped spinach to the skillet and cook until it wilts, about 2-3 minutes, then remove from heat.

In a large bowl, whisk together the eggs and milk, then mix in the Parmesan cheese, oregano, salt, black pepper, and optional red pepper flakes.

Layer half of the zucchini slices in the prepared casserole dish, followed by half of the sautéed spinach mixture and half of the feta cheese.

Repeat the layering with the remaining zucchini slices, spinach mixture, and feta cheese.

Evenly pour the egg-milk mixture over the layered ingredients, ensuring everything is coated.

Sprinkle breadcrumbs evenly over the top of the casserole.

Bake for 30-35 minutes, until the top is golden brown and the center is set.

Allow to cool for a few minutes before slicing into squares or wedges and serving.