Preheat oven to 400° F. Place zucchini in colander; sprinkle with salt. Let stand 10 minutes, then squeeze out moisture as best as you can.

In a large bowl, combine zucchini with eggs, Parmesan and half the mozzarella and cheddar cheeses. Press into a greased 13x9. Bake for 20 minutes.

While the zucchini is cooking, in a large saucepan, cook beef and onion over medium heat until meat is no longer pink. Add the garlic and cook for one minute. Drain. Add the tomato sauce; spoon over zucchini mixture. Sprinkle with remaining cheeses; Bake until heated through, about 10 minutes longer.