Preheat oven 425 degrees.
Cut squash in half lengthwise, then crosswise; scoop out the seeds. Cut lengthwise into ½-inch thick wedges. Toss with onion, 1 Tbsp. 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. Remove from oven and let cool.
Meanwhile, roughly chop almonds and spread on a small baking sheet. Roast in the oven until they begin to smell nutty; then remove and let cool.
In a small bowl, whisk together 2 Tbsp. olive oil, mustard, lemon juice and garlic. Season with salt and pepper. Toss greens with dressing and plate. Top with roasted squash, grapes and almonds.