Veggie Mulligatawny

Name: Veggie Mulligatawny
Yield: Serves 4
Ingredients: 1 celery stick, finely chopped; 1 carrot, peeled and roughly chopped; 1 tomato, roughly chopped; 1 apple, peeled, cored and roughly chopped; 2 stock cubes; 1 tablespoon curry power; salt and freshly ground black pepper; salt and freshly ground black pepper; 1 tablespoon olive oil or butter; 1 tablespoon olive oil or butter; 1 sweet potato, peeled and diced; 1 onion, peeled and diced