Preheat oven to 400°F (200°C).
Place shredded zucchini in a colander and sprinkle with 1/2 teaspoon salt. Let stand for 10 minutes to draw out moisture.
Squeeze out as much moisture as possible from the zucchini.
In a large bowl, combine the zucchini with eggs, Parmesan, and half of the mozzarella and cheddar cheeses.
Press the zucchini mixture evenly into a greased 13x9 inch baking dish.
Bake the zucchini crust for 20 minutes.
While the zucchini bakes, cook ground beef and chopped onion in a large saucepan over medium heat until the meat is no longer pink.
Add minced garlic to the beef mixture and cook for one minute more. Drain any excess fat.
Stir in the tomato sauce with the beef mixture and spoon it over the baked zucchini crust.
Sprinkle the remaining mozzarella and cheddar cheeses over the top.
Bake for an additional 10 minutes, or until the cheese is melted and bubbly.
Let the casserole rest for a few minutes before serving.