Preheat oven to 500°F with rack in upper third position. Using a vegetable peeler, peel skin from 1 squash and discard. Slice the neck of the squash into 1-inch-thick rounds. Slice the bulb in half lengthwise; remove and discard seeds. Slice each half into 1-inch-thick half-moons.
Combine the squash, the melted butter, 1 tablespoon oil, ½ teaspoon salt and ½ teaspoon pepper in a large bowl; toss to coat. Arrange in a single layer in a 9-by-13-inch metal baking pan. (Don't use a glass pan, which could shatter.) Roast, flipping once, until browned around the edges and almost tender, about 25 minutes.
Meanwhile, whisk ⅓ cup broth, 1½ tablespoons maple syrup, 1 tablespoon vinegar and 1 teaspoon mustard together in a glass measuring cup. Carefully add the broth mixture to the pan. Continue roasting until most of the liquid has been absorbed and the squash is very tender, about 8 minutes more.
Transfer the squash to a serving platter. Spoon the remaining pan liquid over the squash. Garnish with thyme, if desired.
