Preheat broiler. Spray a baking sheet with nonstick cooking spray. Place tomatoes on the baking sheet, cut side down. Broil until skins are blistered, about 5 minutes. Set aside to cool. Slip off skins and chop coarsely. Meanwhile, in a medium-sized saucepan, heat oil over medium-low heat. Add onions and saute for 5 minutes. Add garlic and saute until the onions are very soft, about 5 minutes longer. Stir in tomato paste, 2/3 of chopped tomatoes and cook, stirring, for 1 minute. Add 2 tablespoons basil. Transfer the mixture to a food processor or blender and process until smooth; return to the saucepan. Stir in chicken stock and bring to a boil. Reduce heat to low and simmer for 5 minutes. Remove from heat and stir in balsamic vinegar, remaining tomatoes and basil. Season with salt and pepper. Either serve hot or cover and refrigerate until chilled, at least 1 hour. (The soup can be stored, covered, in the refrigerator for up to 2 days.) Makes about 5 cups, serves 6. 90 calories per serving: 4 g protein, 2 g fat, 16 g carbohydrate; 216 mg sodium; 0 mg cholesterol.