Heat the oven to 425°F. Arrange the zucchini on two sheet pans, brush with oil, season with 1/2 teaspoon salt and roast for 15 minutes. Reduce the oven to 375°F.
Brown the beef and onion in a skillet and drain. Add garlic for 30 seconds, then crushed tomatoes, basil, remaining salt and pepper and simmer 8 minutes.
Layer one-third sauce, half the zucchini, one-third mozzarella and Parmesan in a greased 9-by-13-inch dish. Repeat and finish with sauce and remaining cheeses.
Bake uncovered for 35 to 40 minutes, until bubbling and browned in spots.
Rest for 15 minutes before cutting so released juices settle.