Preheat the oven to 400°F. Lightly grease a 9×13-inch or similar baking dish.
Place the sliced zucchini and yellow squash in a colander and sprinkle with the salt. Toss gently and let drain for about 15 minutes.
Pat the zucchini and squash thoroughly dry with paper towels to remove as much excess moisture as possible.
Transfer the vegetables to a large bowl. Add the olive oil, minced garlic, Italian seasoning, and black pepper. Toss until evenly coated.
Arrange half of the zucchini and squash in the prepared baking dish. Sprinkle with half of the mozzarella and Parmesan.
Add the remaining zucchini and squash, then top with the remaining mozzarella and Parmesan.
Sprinkle the panko breadcrumbs evenly over the top.
Bake uncovered for about 25–30 minutes, until the vegetables are tender and the cheese is melted and bubbling. Let rest for 5 minutes before serving.

Drain the vegetables: Salting the zucchini and yellow squash for about 15 minutes helps draw out excess moisture and prevents a watery casserole.

Slice evenly: Cut the zucchini and squash into similar-sized slices so they cook at the same rate.

Cheese: Freshly shredded mozzarella melts especially well. Parmesan adds a savory flavor to the topping.

Panko: Panko breadcrumbs create a lighter, crispier topping than regular breadcrumbs.

Make ahead: Slice and drain the vegetables ahead of time, but assemble the casserole shortly before baking for the best texture.

Storage: Refrigerate leftovers in an airtight container for 3–4 days.

Reheating: Reheat uncovered in the oven or air fryer when possible to help restore the crispy topping.