Preheat the oven to 400 degrees F. Use a knife to lop the top and bottom off the squash. Slice the skin off the sides, then cut the squash in half lengthwise and remove the seeds. Chop into small pieces. Divide the squash between 2 baking sheets.
Trim the Brussels sprouts, then cut them in half if desired (or you can leave them whole). Arrange on the baking sheets with the squash. Break the quartered onions apart and add them to the baking sheets.
Drizzle the vegetables with the olive oil. Season with salt and pepper, sprinkle with the chili powder and toss. Roast 30 to 35 minutes, or until browned.
Take the vegetables out of the oven and tip into a serving dish. Drizzle with the pomegranate molasses and sprinkle on the pomegranate seeds. Toss and serve immediately.