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.
Amy Thielen, author of The New Midwestern Table and host of Heartland Table, says when it comes to freshwater fish, “the lack of a crust opens up a world of flavor possibilities.”