Preheat your oven to 375°F (190°C).

In a large skillet over medium heat, add a drizzle of olive oil. Sauté the chopped onion for about 3-4 minutes until it becomes translucent.

Once the onion is ready, stir in minced garlic and continue cooking for an additional minute.

Crumble the Italian sausage into the skillet. Cook for 5-7 minutes, breaking it apart as it browns.

Stir in diced bell peppers, dried oregano, basil, salt, and pepper. Sauté for about 2-3 minutes.

In a baking dish, layer half of the sliced zucchini at the bottom.

Top the first layer of zucchini with the sausage mixture, spreading it out evenly.

Cover with the remaining sliced zucchini, then generously sprinkle mozzarella and Parmesan cheese on top.

Cover the baking dish with foil and bake for 20 minutes.

Remove the foil and bake uncovered for an additional 10-15 minutes until the cheese is bubbly and golden brown.

Let the casserole rest for at least 5 minutes before serving.