Preheat oven to 350 degrees F. Lightly grease a 2-quart baking dish; set aside. In a large skillet, heat the 2 tablespoons butter and maple syrup over medium-low heat. Add onions; cook about 15 minutes or until very tender and golden, stirring occasionally. Stir in sage, garlic, and the 1/4 teaspoon salt.
Meanwhile, halve squash lengthwise; seed and peel squash. Cut crosswise into 1/4-inch slices. Sprinkle with the 1/2 teaspoon salt and pepper.
Arrange squash in the prepared baking dish. Top with onion mixture and cheese. Bake, covered, for 50 to 55 minutes or until squash is nearly tender.
In a small bowl, combine bread crumbs and hazelnuts; stir in melted butter. Sprinkle over squash mixture. Bake, uncovered, for 10 to 15 minutes more or until squash is tender and crumbs are lightly browned. Let stand for 10 minutes before serving.