Preheat your oven to 375°F (190°C).

Wash and thinly slice the zucchini. You can use a mandoline for consistent slices, but a sharp knife works just as well. Aim for slices about 1/8 inch thick.

Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened, about 5 minutes. Then, add the minced garlic and cook for another minute until fragrant.

Add the sliced zucchini to the skillet with the onion and garlic. Cook, stirring occasionally, until the zucchini is slightly softened but still has some bite, about 8-10 minutes. Don't overcook it at this stage; it will continue to cook in the oven.

Season the zucchini mixture with salt, pepper, and Italian seasoning. Stir well to combine. Taste and adjust the seasoning as needed.

In a medium bowl, combine the ricotta cheese, beaten egg, and grated Parmesan cheese. Mix well until smooth and creamy.

Lightly grease a 9x13 inch baking dish.

Spread a thin layer of the ricotta cheese mixture on the bottom of the dish.

Arrange a layer of the sautéed zucchini mixture over the ricotta cheese. Try to cover the entire bottom of the dish evenly.

Sprinkle about 1/3 of the shredded mozzarella cheese over the zucchini layer.

Repeat the layers of ricotta cheese mixture, zucchini mixture, and mozzarella cheese two more times, ending with a layer of mozzarella cheese on top.

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

Remove the foil and bake for another 20-25 minutes, or until the cheese is melted and bubbly and the casserole is heated through. The top should be golden brown and slightly crisp.

Remove the casserole from the oven and let it rest for 10-15 minutes before serving.

Garnish with fresh chopped basil or parsley, if desired. Serve warm and enjoy!