Cook the sweet potatoes until just cooked through. Bake, microwave or boil. Set aside to cool.
Meanwhile heat oil in large saucepan or Dutch oven over medium high heat. Add onion and cook, until it just begins to brown, 2-4 minutes. Add garlic and cook 1 minute more. Stir in juice, chilies, ginger and allspice. Simmer for 10 minutes.
Peel the sweet potatoes and chop into bite-size pieces. Add half to the pan. Blend the other half along with the broth and peanut butter. Puree until completely smooth. Add the puree to the pan and stir well to combine. Thin the bisque with water if needed. Season with pepper. Heat until hot. Garnish with cilantro if desired.