Preheat the oven to 400 degrees F (200 degrees C). Place the sweet potatoes directly on the rack and bake until tender enough to easily pierce with a fork, about 45 minutes. Remove from the oven and allow to cool.
Heat the oil in a large saucepan or soup pot over medium heat. Add the onion and ginger; cook and stir until tender, about 5 minutes. Stir in the curry paste and heat for 1 minute, then whisk in the coconut milk and vegetable broth. Bring to a boil, then reduce heat to low and simmer for about 5 minutes.
Remove the skins from the sweet potatoes and cut into bite size chunks. Add to the soup and cook for 5 more minutes so they can soak up the flavor. Stir in lemon juice and season with salt. Ladle into bowls and garnish with a sprinkle of the sesame seeds and cilantro.