3 cups reduced-sodium tomato-vegetable juice blend or tomato juice
1 4-ounce can diced green chiles, preferably hot, drained
2 teaspoons minced fresh ginger
1 teaspoon ground allspice
1 15-ounce can vegetable broth
Prick sweet potatoes in several places with a fork. Microwave on High until just cooked through, 7 to 10 minutes. Set aside to cool.
Meanwhile, heat oil in a large saucepan or Dutch oven over medium-high heat. Add onion and cook, stirring, until it just begins to brown, 2 to 4 minutes. Add garlic and cook, stirring, for 1 minute more. Stir in juice, green chiles, ginger and allspice. Adjust the heat so the mixture boils gently; cook for 10 minutes.
Meanwhile, peel the sweet potatoes and chop into bite-size pieces. Add half to the pot. Place the other half in a food processor or blender along with broth. Puree until completely smooth. Add the puree to the pot and stir well to combine. Thin the bisque with water, if desired. Season with pepper. Heat until hot. Garnish with cilantro, if desired.