Preheat oven to 375F (if you have a "browning" setting on your oven, use that).
Slice all of the squash into rounds, about a quarter of an inch thick.
Layer the cut squash in an oven safe baking dish. Season each layer with salt and pepper and dot with pieces of butter.
In a cup, use a fork to whisk together egg and most of the cheese. Pour the mixture over the prepared squash. Top with remaining cheese and sprinkle with panko crumbs.
Bake until brown and bubbly.
Recipe by Saffron and Honey at https://saffronandhoney.com/summer-squash-gratin/