Preheat your oven to 375°F (190°C). While the oven is heating up, grease a 1 ½-quart baking dish to prevent sticking, and set it aside for later use.

Place the sliced zucchini in a colander and sprinkle it with salt. Allow the zucchini to sit for about 10 minutes to help draw out excess moisture. After this time, squeeze out as much water as possible using a dish towel. Wrap the zucchini in the towel, and wring it multiple times to ensure you extract most of the liquid. This step is important to prevent your casserole from turning out watery.

In a large mixing bowl, combine the squeezed zucchini with chopped onion, minced garlic, beaten eggs, ½ cup Parmesan cheese, mozzarella cheese, and cheddar cheese. Once mixed, gently press the combined ingredients evenly into the prepared baking dish.

Place the baking dish, uncovered, into the preheated oven and bake for 20 minutes. This initial bake helps set the casserole.

While the casserole is baking, prepare the topping. In a small bowl, pour melted butter over breadcrumbs and mix in 2 tablespoons of Parmesan cheese. Stir until the breadcrumbs are fully coated and combined with the butter.

After baking for 20 minutes, carefully remove the casserole from the oven. Evenly sprinkle the buttered breadcrumbs over the top of the zucchini mixture. Return the dish to the oven and continue baking for an additional 5-10 minutes, or until the topping is golden brown and crispy.

Once baked to perfection, remove the casserole from the oven and allow it to cool slightly before serving. Enjoy your delicious, crispy zucchini casserole!