Shred the zucchini and place it in a colander, then sprinkle with salt and let it sit for 10 minutes—this draws out excess moisture that would make your crust soggy. After 10 minutes, squeeze the zucchini firmly with your hands or wrap it in a clean kitchen towel and press out as much liquid as possible. In a bowl, combine the squeezed zucchini with eggs, Parmesan cheese, and half of the mozzarella and cheddar. Mix until everything is evenly incorporated, then press this mixture firmly into a 9x13 inch baking dish, creating an even layer that will serve as your crust.

Preheat your oven to 400°F if you haven't already. Bake the pressed zucchini mixture for 20 minutes until it's set and beginning to firm up. While the crust bakes, you'll have time to prepare the meat sauce in the next step, keeping your workflow efficient.

While the crust bakes, heat a large skillet over medium-high heat and add the ground beef along with the diced onion. Cook, stirring occasionally and breaking up the beef as it browns, until the meat is fully cooked and the onion is softened, about 8-10 minutes. Drain off any excess fat by pouring the mixture through a colander or pushing it to the side of the pan and tilting to drain the grease. Return the drained beef and onion to the skillet, then stir in the Italian seasoning and garlic powder to bloom the spices for about 30 seconds. Pour in the tomato sauce and stir to combine, creating a rich, seasoned sauce.

Once the zucchini crust has baked for 20 minutes and is set, remove it from the oven and pour the beef and tomato sauce mixture evenly over the top. I like to use a spatula to spread it in an even layer so every bite gets some sauce. Sprinkle the remaining mozzarella and cheddar cheese over the sauce, then scatter the diced green pepper pieces across the top for color and freshness.

Return the casserole to the 400°F oven and bake for 20 minutes until the cheese is melted and bubbly and the edges are lightly golden. Let the casserole rest for 5 minutes out of the oven before serving—this helps the crust set slightly so it holds together when you cut into it.