Heat the oil in a large pot over medium-high heat. Add the onion, cover, and cook until softened, about 5 minutes. Stir in the ginger, then add the potato, tomatoes, and vegetable broth and bring to a boil. Reduce bthe heat to medium, add the corn, and season to taste with salt and pepper. Simmer until the vegetables are tender, about 15 minutes. Stir in the coconut milk and basil. To serve, ladle into bowls and drizzle a small amount of siracha on top of each bowl if desired.