Preheat the oven and prepare the baking dish - Preheat the oven to 350°F. Grease a 9x13 inch baking dish with cooking spray or line it with aluminum foil for easy cleanup.

Sauté the vegetables - In a large skillet over medium heat, warm up the olive oil. Add the onion and garlic and cook for 2-3 minutes until softened. Add the spinach and mushrooms and cook for another 5-7 minutes until the vegetables are wilted and tender. Remove from heat and let cool slightly. - Make sure to use a large skillet so that all the vegetables can fit comfortably. You can also add some salt and pepper for seasoning.

Prepare the egg mixture - In a mixing bowl, whisk the eggs and milk together until well combined. Add the Parmesan cheese and mix again. - For a lighter version of the dish, you can use low-fat milk or unsweetened non-dairy milk instead of whole milk.

Combine the vegetables and the egg mixture - Add the sautéed vegetables to the egg mixture and stir well to combine. Season with salt and black pepper to taste. Pour the mixture into the prepared baking dish. - Make sure the vegetables are not too hot when you add them to the egg mixture, as they may cook the eggs prematurely and result in a watery casserole.

Bake in the oven - Bake the casserole in the preheated oven for 30-35 minutes, or until the top is golden brown and the center is set. Remove from the oven and let cool for a few minutes before serving. - If the casserole is browning too quickly, cover it with aluminum foil and continue baking until cooked through.