Melt the butter in a pan and sweat the onion for 5 minutes. Stir in the curry powder and cook for a further 30 seconds or so. Add the stock, bananas, lemon juice, and salt to taste. Bring to the boil, cover and simmer gently for 15 minutes. Process the soup in a processor or blender and stir in the cream. Taste and adjust seasonings, then return the soup to the pan and reheat to serve.