Preheat oven to 350 degrees F (175 degrees C).
Place bread cubes in a medium bowl and pour melted butter over the bread. Add the zucchini, onion, garlic salt and egg; mix well. Transfer the mixture into a 9×13 inch baking dish and top with the cheese.
Bake, covered, in preheated oven for 30 minutes. Then uncover the dish and bake for another 30 minutes.