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

Thinly slice the zucchinis and 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 chopped onion and minced garlic, sautéing until fragrant. Add the chopped spinach and cook until wilted. Remove from heat and let cool slightly.

In a large bowl, combine the Greek yogurt, eggs, dried oregano, dried basil, salt, and pepper. Stir in the sautéed spinach mixture, crumbled feta cheese, and grated Parmesan cheese. Add the sliced zucchini and mix until well combined.

Transfer the mixture to the prepared baking dish, spreading it out evenly. Top with a sprinkle of additional Parmesan cheese if desired.

Bake in the preheated oven for 30-35 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.