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 9x13 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.