From Fine Cooking Magazine - Issue No. 17.
To make butternut squash easier to handle, cut off the neck, peel it, and cut it into pieces to use in this gratin. Reserve the bottom of the squash, which contains the seed cavity, for roasting.
Heat the oven to 350 degrees F. Lightly coat a gratin dish with butter. In a large skillet over low heat, melt the butter with the oil. Add the onions and the sage. Cover and cook over low heat while you prepare the squash.
Put the squash in a steaming basket and set over simmering water. Cover and let steam until the squash is tender, about 10 minutes. Put the squash in the prepared dish with the garlic and 3/4 teaspoon salt. Toss gently to combine.
Continue to cook the onions, stirring frequently, until they caramelize, about 25 minutes total. Season with salt and pepper and add them to the gratin dish, mixing them with the squash. Pour the warm cream over the vegetables. Toss the breadcrumbs with the melted butter and sprinkle the crumbs evenly over the gratin. Bake until the top is browned and bubbling, about 40 minutes.