Preheat your oven to 375°F (190°C).

In a large skillet, heat olive oil over medium heat. Add diced onion and sauté for 3–4 minutes until translucent.

Add minced garlic to the skillet, sauté for an additional minute, stirring constantly.

Crumble the Italian sausage into the skillet and cook for 5–7 minutes until browned.

Stir in diced bell peppers, dried oregano, and basil; cook for 2–3 minutes until tender. Season with salt and black pepper.

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

Spread the sausage mixture over the layer of zucchini.

Add the remaining slices of zucchini on top of the sausage mixture.

Sprinkle mozzarella and Parmesan cheese over the top.

Cover with aluminum foil and bake for 20 minutes.

Remove the foil and bake for an additional 10–15 minutes until cheese is golden.

Let the casserole rest for 5 minutes before serving.