Prepare the grill for direct cooking over medium heat (350° to 450°F).
In a large stockpot combine the chicken broth and bread. Set aside while you prepare and grill the vegetables.
Flatten the bell pepper half with the palm of your hand. Brush the pepper half, tomato halves, and onion slices with oil. Brush the cooking grates clean. Place the pepper and tomato halves, skin side down, and the onion slices over direct medium heat, close the lid, and cook until lightly charred, turning once. The pepper and tomatoes will take 6 to 8 minutes and the onion will take 8 to 12 minutes. Remove from the grill as they are done. When the pepper and tomatoes are cool enough to handle, remove their skins. Coarsely chop all the grilled vegetables.
Flatten the bell pepper half with the palm of your hand. Brush the pepper half, tomato halves, and onion slices with oil. Brush the cooking grates clean. Place the pepper and tomato halves, skin side down, and the onion slices over medium heat, close the lid, and cook until lightly charred, turning once. The pepper and tomatoes will take 6 to 8 minutes and the onion will take 8 to 12 minutes. Remove from the grill as they are done. When the pepper and tomatoes are cool enough to handle, remove their skins. Coarsely chop all the grilled vegetables.
Add the grilled vegetables, the chopped basil, celery, garlic, salt, and pepper to the stockpot. Bring to a boil over high heat on the stove, and then reduce the heat to low and simmer for 45 minutes, stirring occasionally to help the bread dissolve and thicken the soup. Ladle into bowls, top with the basil chiffonade, and serve hot.
