Place a large rimmed baking sheet in oven and preheat oven to 450°F. Toss together squash, parsnips, Brussels sprouts, onion, 2 tablespoons oil, and 1 ½ teaspoons salt in a large bowl. Carefully spread vegetable mixture on preheated baking sheet. Return to oven and roast, tossing halfway through, until tender and browned, about 20 minutes. Let cool on baking sheet for 5 minutes.
Meanwhile, cook quinoa according to package directions. Transfer to a large bowl. Add spinach and remaining ½ teaspoon salt. Toss until spinach is slightly wilted.
Whisk vinegar and mustard in a small bowl. Gradually drizzle in remaining ¼ cup oil, whisking constantly. Stir 2 tablespoons vinaigrette into quinoa.
Divide quinoa among 4 bowls. Top with roasted vegetables. Dollop with hummus and drizzle with remaining vinaigrette. Serve warm or at room temperature.
