Preheat oven to 400° F. Put the sweet potatoes on an oven rack and bake for 50 minutes or until tender. Remove from oven and let cool. Heat the vegetable oil in a large saucepan over medium heat. Sauté the onion and ginger until the onion softens, about 5 minutes. Stir in the curry paste and cook for 1 minute. Add the coconut milk and broth and gently bring to a boil. Reduce heat to a simmer, partially cover, and cook for 5 minutes. Skin the potatoes and cut into 1-inch chunks. Add the potatoes to the soup and simmer for 5 minutes. Stir in the lemon juice and salt. Ladle the soup into serving bowls. Drizzle the sesame oil evenly over the servings. Garnish with the cilantro.