Slice zucchini; salt and let sit for 15 minutes, then pat dry.
Cook beef, onion, and garlic for 5–7 minutes; drain fat.
Add marinara, oregano, salt, and pepper; simmer for 5 minutes. Taste and adjust.
Mix ricotta, egg, Parmesan, basil, and 1 cup mozzarella.
Layer sauce, zucchini, and ricotta in a 9x13-inch dish; end with sauce and 1 cup mozzarella.
Bake at 375°F covered for 30 minutes, then uncovered for 10–15 minutes.
Rest for 10 minutes; serve in a white ceramic dish with basil.