4 pounds winter squash, such as buttercup or kabocha—halved lengthwise,
seeded and cut into 2-inch-wide wedges
6 tablespoons unsalted butter, melted, plus 2 tablespoons softened
Salt and freshly ground pepper
8 thyme sprigs
4 large sweet onions, such as Walla Walla, halved lengthwise and sliced
crosswise 1/4 inch thick
2 pounds assorted mushrooms, such as shiitake, cremini and Portobello, stems
discarded and caps quartered
1/2 cup dry red wine
1 cup chicken stock or canned low-sodium broth
Preheat the oven to 425°. Put the squash wedges on a large, rimmed baking sheet and brush all over with 2 tablespoons of the melted butter. Season with salt and pepper and scatter 4 of the thyme sprigs on top. Bake in the upper third of the oven for about 25 minutes, or until just tender. When cool enough to handle, peel the squash and cut the flesh into 1-inch dice. Transfer to a large baking dish.
Meanwhile, in a large roasting pan, toss the onions and mushrooms with the remaining 4 tablespoons of melted butter and the red wine. Season with salt and pepper and top with the remaining 4 thyme sprigs. Cover with foil and bake in the lower third of the oven for about 30 minutes, or until the onions have softened and the mushrooms have released their liquid.
Set a large strainer over a medium bowl and carefully pour in the liquid from the roasting pan; you should have about 1 1/2 cups.
Stir the 2 tablespoons of softened butter into the onions and mushrooms in the roasting pan. Roast uncovered for about 1 hour, or until the onions and mushrooms are very tender and deeply browned around the edges. Add the vegetables to the squash in the baking dish, stir to combine and season with salt and pepper. Cover with foil to keep warm.
Put the roasting pan over 2 burners set on moderately high heat. When the pan starts sizzling, pour in the chicken stock and use a wooden spoon to scrape up the browned bits on the bottom. Boil the stock for 1 minute, then strain it into the onion and mushroom cooking liquid; set the 2 1/3 cups of cooking liquid aside for making the gravy for the Beef Tenderloin with Roasted Garlic Crust. Serve the vegetables hot.
The vegetables and their cooking liquid can be refrigerated separately for up to 2 days. Reheat the vegetables, covered, in a 425° oven.