Preheat oven to 400 degrees. Place sweet potatoes on a baking sheet and roast for 1 hour, turning once, or until potatoes are very soft. When cool enough to handle, scoop cooked potato out of the skins. Discard skins.
Meanwhile, in a 5-quart Dutch oven or large soup pot, heat oil over medium. Add onion, season with salt and pepper, and cook, stirring occasionally, until soft and caramelized, about 20 minutes. Add bell pepper, jalapenos, and garlic; cook for 5 minutes. Add sherry (if using), sweet potato, and broth; stir to combine. Bring to a boil, reduce to a simmer, and cook for 20 minutes, until flavors have blended.
Working in batches, puree soup in a blender. Combine batches in a clean pot. The soup should be thick. (If you prefer a thinner soup, add water or a little more broth.) Taste and adjust seasoning.
In a small saucepan over medium heat, bring vinegar, honey, and curry powder to a boil. Add apple; stir for 1 minute and remove from heat. To serve, ladle the soup into warm bowls, add a spoonful of apples, a spoonful of yogurt, and a sprinkle of mint.