In a large saucepan, cover the sweet potatoes and lemon grass with water and bring to a boil. Reduce the heat and simmer until the potatoes are fully cooked, about 20 minutes. Remove the lemon grass stalks. Drain the sweet potatoes, reserving the liquid and the potatoes separately.
In a sauté pan, heat the oil over medium heat until it ripples. Add the onion and cook until soft, about 5 minutes. Add the garlic and cook for 2 minutes. Add the jalapeño and minced ginger and sauté for 3 minutes.
Combine the cooked sweet potatoes, cooked onion mixture, coconut milk, and 3 cups of the sweet potato cooking liquid in a food processor or blender. Process until smooth, about 30 seconds.
Pour the soup into a nonreactive saucepan over medium heat and bring to a simmer. Stir in the lime juice, salt, and cayenne pepper. If the soup is too thick, add more of the sweet potato cooking liquid. Serve the soup hot or refrigerate until well chilled. Top with Peanut Pesto.
½ cup unsalted peanuts, toasted
2 tablespoons chopped fresh basil leaves
2 tablespoons chopped fresh mint leaves
1 tablespoon chopped fresh cilantro leaves and stems
Crush the peanuts and combine with the basil, mint, and cilantro in a small bowl and toss well. When serving the soup, sprinkle each serving with about 1 tablespoon of the peanut pesto.
Tip: To crush the peanuts place them on a cutting board or work surface and set a heavy saucepan on top. Press down on the pan to break the peanuts into small, irregular pieces.