Preheat an oven to 350 degrees F (175 degrees C). Grease a 2-quart baking dish.
Arrange the zucchini slices, tomato slices, and onion slices in alternating layers in the prepared baking dish. Melt the butter in a saucepan over medium-low heat; stir the vinegar, garlic salt, basil, and mustard into the melted butter. Pour the butter mixture over the vegetables; sprinkle the Parmesan cheese over the dish.
Bake in the preheated oven until thoroughly heated, about 30 minutes.
