Preheat the oven to 450 degrees. Put the sweet potatoes, apples, onions and garlic in a roasting pan. Toss them with the oil and a few shakes of salt and pepper (optional).
Roast, tossing every 10 minutes, until they are soft, about 30 minutes.
Puree the vegetable/apple mixture in a blender or food processor (or in the pot using an immersion blender), adding just enough broth to cover it. Add more broth to the blender until the soup reaches the desired consistency, so it is smooth and not too thick. If you are using a blender, you will probably need to puree the soup in two batches.
Warm the soup over low heat in a stockpot until ready to serve, refrigerate it for up to 1 day, or freeze it for up to 3 months. Stir in sour cream at the table for a creamier taste, if desired.