1. Preheat oven to 375°F (190°C) and grease a 9×13-inch baking dish.

2. Place sliced zucchini in a colander, sprinkle with salt, let sit 20 minutes, then pat dry.

3. Whisk together heavy cream, salt, pepper, and chopped herbs in a bowl.

4. Layer half the zucchini and onion in the dish, pour half the cream mixture over, then sprinkle with half the cheese.

5. Repeat layers with remaining zucchini, onion, cream mixture, and cheese.

6. Combine breadcrumbs with melted butter or olive oil in a small bowl and spread evenly over the top.

7. Cover dish with foil and bake for 25 minutes.

8. Remove foil and bake an additional 15 minutes until topping is golden and crisp.

9. Let casserole rest 5–10 minutes before serving.