Cut squash in half lengthwise, then crosswise; scoop out the seeds. Cut lengthwise into 1/2-inch-thick wedges. Toss with onion, 1/2 tablespoon oil and salt in a large bowl. Spread in an even layer on a baking sheet.
Roast, stirring once or twice, until tender and beginning to brown, about 30 minutes.
Combine the remaining 1/2 tablespoon oil, rosemary, syrup and mustard in a small bowl. Toss the vegetables with the dressing.