How to Make It
Heat oven to 400° F. On a rimmed baking sheet, toss the squash with 2 tablespoons of the oil, ½ teaspoon salt, and ¼ teaspoon pepper. Roast, tossing once, until golden brown and tender, 30 to 35 minutes. Let cool slightly.
Meanwhile, spread the hazelnuts on a second rimmed baking sheet and toast in oven, tossing occasionally, until fragrant, 10 to 12 minutes. Let cool slightly, then rub in a clean dish towel to remove the skins (discard skins). Roughly chop.
In a large bowl, whisk together the vinegar, mustard, the remaining 3 tablespoons of oil, and ¼ teaspoon each salt and pepper. Add the squash, hazelnuts, greens, and onion and toss to combine. Sprinkle with the blue cheese.