Slice the zucchinis into thin rounds and sprinkle with a little salt. Let sit for 10 minutes to draw out excess moisture, then pat dry with a paper towel.
In a mixing bowl, combine shredded cheddar, heavy cream, minced garlic, chopped herbs, and sautéed onions. Mix until smooth and creamy, then season with salt and pepper to taste.
Add the drained zucchini slices to the bowl and gently fold them into the cheese mixture until evenly coated.
Transfer the zucchini mixture into the prepared baking dish, spreading it out evenly.
Sprinkle toasted panko breadcrumbs over the top, followed by a little extra shredded cheese if desired.
Place the dish in a preheated oven at 180°C (350°F) and bake uncovered for 35-40 minutes, until the top is golden brown and bubbling around the edges.
Once baked, remove the casserole from the oven and let it rest for 5 minutes to set before slicing.
Serve warm, enjoying the cheesy, tender zucchini with a crispy topping and fragrant herbs.
