Position a rack in your oven 6 inches from the broiler and heat the broiler on high.
Prepare the Baking Dish:
Grease a shallow 9×13-inch baking dish or rimmed sheet pan with softened butter.
Season and Broil the Zucchini:
Season the thinly sliced zucchini with kosher salt and freshly ground black pepper. Arrange the slices on the prepared baking pan. Broil just until the zucchini begins to soften, about 4 minutes.
Prepare the Cheese Sauce:
While the zucchini is broiling, whisk together the heavy cream, dried thyme, garlic powder, freshly grated Parmigiano-Reggiano, and all-purpose flour in a small bowl.
Assemble the Dish:
Pour the cheese sauce mixture evenly over the broiled zucchini. Top with freshly grated Gruyere cheese and panko breadcrumbs for added crunch.
Broil Until Golden:
Return the baking pan to the oven and broil until the top is golden brown and bubbly, about 4-5 minutes.
Serve and Enjoy:
Serve the cheesy scalloped zucchini hot as a delicious side dish.