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

Heat olive oil in a large skillet over medium heat, add chopped onion, and sauté for about 4 minutes until translucent.

Add minced garlic to the skillet and cook for an additional minute, then incorporate the chopped spinach and cook for 2-3 minutes until wilted.

In a mixing bowl, whisk together eggs, milk, grated Parmesan, dried oregano, salt, and black pepper.

Layer half of the sliced zucchini in the prepared dish, followed by half of the sautéed spinach mixture and half of the crumbled feta.

Repeat layering with remaining zucchini, spinach, and feta, then pour egg-milk mixture evenly over the vegetables.

Sprinkle breadcrumbs over the top of the casserole for a crispy finish.

Bake in the preheated oven for 30-35 minutes until the top is golden brown and the center has set.

Allow to cool for 5-10 minutes before serving.