Preheat your oven to 180°C (350°F). Lightly grease your baking dish to prevent sticking.
Using a mandoline or sharp knife, slice the zucchinis into thin rounds about 1/8 inch thick. This helps them layer evenly and bake crisp on the edges.
In a sauté pan, heat a tablespoon of olive oil over medium heat. Add the chopped onion and cook until it turns golden and fragrant, about 5 minutes. This sweetens the onion and adds flavor to your casserole.
In a mixing bowl, combine the shredded cheese, heavy cream, sautéed onions, chopped herbs, and season with salt and pepper. Stir until smooth and creamy, creating a rich sauce that will coat the zucchini layers.
Arrange a layer of zucchini slices in the prepared dish, slightly overlapping for a neat, layered look. Season lightly with salt and pepper as you go.
Pour about one-third of the cheese mixture evenly over the zucchini layer, ensuring all slices are coated. Repeat layering with remaining zucchini and cheese mixture until all ingredients are used.
Sprinkle the toasted breadcrumbs evenly over the top for a crispy, golden crust. Add a little extra cheese on top if you like it extra cheesy.
Bake the casserole in the preheated oven for 35-40 minutes, until it’s bubbling around the edges and the top is golden brown. The zucchini should be tender and the cheese melted and gooey.
If the top browns too quickly, tent the dish loosely with aluminum foil and continue baking. For an extra crispy top, broil for 2 minutes, watching carefully to prevent burning.
Remove the casserole from the oven and let it rest for about 10 minutes. This helps it set, making it easier to slice and serve without falling apart.
Slice into squares and serve warm, garnished with extra herbs if desired. Enjoy the combination of tender zucchini, melted cheese, and crispy topping in every bite!
