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

Heat about 2 tablespoons of olive oil over medium heat and sauté the chopped onions for approximately 4 minutes until translucent.

Stir in the minced garlic and sauté for about 1 minute.

Mix in the chopped spinach and sauté for 2-3 minutes until wilted.

In a mixing bowl, combine the eggs, milk, grated Parmesan cheese, dried oregano, salt, and pepper.

Layer half of the sliced zucchini at the bottom of the casserole dish, followed by half of the sautéed spinach mixture and half of the crumbled feta.

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

Pour the whisked egg and milk mixture evenly over the layered ingredients.

Sprinkle breadcrumbs over the top of the casserole.

Bake for 30-35 minutes until the top is golden and edges are bubbling.

Allow to cool for a few minutes before serving.