Position a rack in your oven 6 inches from the broiler and heat broiler on high.
Grease a shallow 9×13-inch baking dish or rimmed sheet pan with softened butter.
Season zucchini with salt and pepper and arrange on baking pan. Broil just until squash is beginning to soften, about 4 minutes.
While squash bakes, whisk together cream, thyme, garlic powder, parmesan cheese, and flour in a small bowl. Pour mixture evenly over squash, then top with Gruyere cheese and panko breadcrumbs.
Return to oven and broil until golden brown and bubbly, 4-5 minutes. Enjoy!
