1. Place shredded zucchini in a colander over the sink. Sprinkle with 1 tsp salt, toss to coat, and let sit 10 minutes to draw out moisture. Transfer to a clean kitchen towel and squeeze very firmly until no liquid drips out — do this twice if needed. The zucchini reduces dramatically in volume. This step determines whether your crust holds together.

2. Preheat oven to 400°F. Spray a 9×13-inch baking dish with cooking spray. In a large bowl, combine the wrung-out zucchini, beaten eggs, Parmesan, and half the mozzarella and cheddar. Mix thoroughly. Press the mixture firmly and evenly into the prepared baking dish. Bake 20 minutes until set and lightly golden around the edges.

3. While the crust bakes, cook ground beef and diced onion in a large skillet over medium heat, breaking meat into small crumbles. Drain any excess fat once fully cooked. Add garlic and cook 1 minute. Stir in tomato sauce, Italian seasoning, and garlic powder. Season with salt and pepper. Simmer 3–4 minutes.

4. Remove the baked crust from the oven. Spoon the meat sauce evenly over the entire surface. Scatter remaining mozzarella and cheddar across the top. Add any optional toppings now.

5. Return the casserole to the oven and bake another 10–15 minutes until cheese fully melts and starts to bubble. Switch oven to broil for 1–2 minutes to get golden, bubbly cheese on top — watch constantly so it doesn’t burn.

6. Let the casserole rest 5–10 minutes before slicing. Use a thin sharp spatula and start from a corner for the cleanest slices.