Mac-and-cheese—essentially a gratin made with pasta—is one of America’s favorite foods. So why not sweet butternut squash in a creamy sauce with a toasted breadcrumb topping? It’s festive enough for the Thanksgiving table.
Preheat oven to 350° and set an oven rack to the middle position. In a large bowl, whisk together cream and broth, then add squash, cheese, 1 teaspoon salt, and ½ teaspoon pepper. Toss well. Pour into a gratin or baking dish, cover with foil, and bake until squash is tender, 35 to 45 minutes, turning the dish and removing the foil halfway through. Remove from oven and set aside. Leave the oven on.
Meanwhile, melt 3 tablespoons butter in a skillet over medium-high heat, add onion, and cook, stirring occasionally, until it begins to brown, about 10 minutes. Add rosemary, remaining ¼ teaspoon salt, and ¼ teaspoon pepper; cook until fragrant, about 1 minute. Spread evenly over squash. Turn the broiler to high.
In a food processor, pulse bread with remaining 1 tablespoon butter, garlic, and nutmeg to create coarse breadcrumbs. Sprinkle over squash. Broil, uncovered, until topping is golden brown, 5 to 7 minutes. Let rest at least 20 minutes before serving.