Salt the zucchini: Toss sliced zucchini with 1/2 teaspoon salt, let drain 15 minutes, then pat very dry.

Brown the beef: Cook ground beef in a skillet over medium-high heat until no pink remains, about 6 minutes. Drain off excess fat, leaving a tablespoon. Add onion and cook until soft, 4 minutes, then add garlic and cook 30 seconds.

Simmer the sauce: Stir in tomato sauce, Italian seasoning, remaining 1/2 teaspoon salt, and pepper. Bring to a simmer and cook 5 minutes, stirring occasionally.

Layer the casserole: Preheat oven to 375°F (190°C). Spread a thin layer of sauce in a 9x13-inch dish. Layer half the zucchini, half the remaining sauce, and 1 cup mozzarella. Repeat with remaining zucchini, sauce, and top with the last 1 cup mozzarella and all of the Parmesan.

Bake and broil: Bake for 25 minutes until bubbly, then broil 2 to 3 minutes until the top is golden and crisp. Let rest 5 minutes, then scoop onto plates and top with fresh basil.