In a large saucepan, heat the vegetable oil. Add the sliced onion, ginger, tomatoes, curry powder and tomato paste and cook over moderate heat, stirring, until softened, about 7 minutes. Stir in the broth, coconut milk and banana and bring to a boil. Season with salt and cook over moderate heat until the banana is very tender, about 30 minutes. Add the chopped chicken and cilantro and simmer until just heated through. Ladle into bowls and serve.