Preheat oven to 375 degrees. Toss prepared vegetables in a very large bowl. Add the olive oil, and a healthy amount of salt and pepper. Mix until all vegetables are completely coated with oil and seasoning.
Transfer the vegetables and the oil to two separate baking sheets. Roast in the oven for an hour, rotating rakes half way.
Once roasted transfer the vegetables, including the juices and oil, to a large bowl and puree with am emersion blender until smooth (or do in batches in a food processor.).
Stir in the vegetable broth, basil, and salt and pepper to taste.