Brush skin side of peppers with 1 tablespoon oil. Bake peppers, skin side up, on lightly greased aluminum foil-lined baking sheets at 400° for 45 minutes or until peppers look blistered.
Place peppers in a large zip-top plastic freezer bag; seal and let stand 20 minutes. Peel peppers; remove and discard seeds.
Sauté onions in remaining 2 tablespoons hot oil in a Dutch oven over medium-high heat 10 minutes or until tender; add garlic, and cook 1 more minute. Add vegetable broth and peppers, and bring to a boil; reduce heat to medium, cover, and simmer, stirring occasionally, 30 minutes.
Remove from heat; let stand 15 minutes. Process mixture, in batches, in a food processor or blender until smooth.
Return pureed mixture to Dutch oven; cook over medium heat 5 minutes. Stir in basil and next 3 ingredients, and cook 5 more minutes or until thoroughly heated. Stir in lemon juice. Serve with sour cream.