Preheat oven to 400. In a large roasting pan combine onion, carrots, parsnips and squash. Coat with cooking spray and roast for 30 minutes.
Place veggies in a large pot. Add broth and milk. Season to taste with salt and pepper. Cook over medium heat for 10 minutes. Puree soup in pot using an immersion blender or transfer to blender or food processor (in batches if necessary) and puree till smooth. Yields eight 1 1/2 cup servings.