Preheat oven to 425°F. Toss squash and onion with 2 tablespoons oil on two rimmed baking sheets. Season with salt and pepper. Roast, rotating pans once, until golden brown and tender, 20 to 25 minutes.
Meanwhile, bring cider to a boil in a small saucepan over high heat. Reduce heat to low and gently simmer until liquid has reduced to 3 tablespoons, 12 to 15 minutes.
Whisk together mustard, vinegar, reduced cider, and remaining 1 tablespoon oil in a bowl. Season with salt and pepper. Stir in parsley.
Drizzle squash and onion with vinaigrette just before serving.
