Preheat your oven to 375°F (190°C) and grease a 1 ½-quart baking dish with butter or olive oil.

Slice zucchini into thin rounds, sprinkle with salt, and let drain in a colander for about 10 minutes.

Gather zucchini slices in a dish towel and gently squeeze to remove excess moisture.

In a large mixing bowl, combine the zucchini, diced onion, minced garlic, and beaten eggs. Stir in the grated cheeses until well mixed.

Press the mixture evenly into the prepared baking dish and bake uncovered for 20 minutes.

Melt butter in a small bowl, then mix in panko crumbs and additional Parmesan cheese.

After the initial baking, carefully sprinkle the breadcrumb mixture over the top of the casserole and return to the oven for an additional 5-10 minutes.

Allow the casserole to cool for a few minutes before slicing and serving.