Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with olive oil or cooking spray.

Thinly slice the zucchinis into rounds. Place them in a colander, sprinkle with salt, and let them sit for 10 minutes to draw out excess moisture. Pat dry with a paper towel.

In a large skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for 1 minute until fragrant. Add the chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat and set aside.

In a large bowl, combine the Greek yogurt, eggs, dried oregano, salt, and black pepper. Stir in the crumbled feta cheese and grated Parmesan cheese. Add the sautéed spinach and sliced zucchini, mixing until everything is well coated.

Transfer the mixture to the prepared baking dish, spreading it out evenly. Top with an extra sprinkle of feta cheese if desired.

Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is set.

Let the casserole cool for a few minutes before slicing and serving. Enjoy your delicious and healthy baked zucchini casserole!