Preheat oven to 350°F. Butter a 9-inch baking dish with 1 tablespoon butter. Peel and seed squash, then thinly slice using a hand slicer, such as a Benriner or Mandoline slicer. Layer squash in baking dish, sprinkling some of salt and pepper between each layer. When baking dish is half filled, sprinkle 1/2 of cheese and thyme over squash. Continue layering squash with salt and pepper. Pour cream evenly over top layer, then sprinkle with remaining cheese and thyme. Cover baking dish with foil and bake until squash is tender, about 45 minutes. Remove foil from baking dish and preheat broiler. Broil gratin until top is browned about 4 minutes. Serve.
"Vegetables are perishable, so we don't have any indication of what they looked like 500 years ago," says James Nienhuis, a professor in the Department of Horticulture at the University of Wisconsin.