Minced beef & sweet potato stew

Name: Minced beef & sweet potato stew
Prep time: 20 min
Cook time: 1 h
Total time: 1 h 20 min
Cuisine: British
Ingredients: 1 tbsp sunflower oil; 1 large onion , chopped; 1 large carrot , chopped; 1 celery stick, sliced; 500g lean minced beef; 1 tbsp tomato purée; 1 tbsp mushroom ketchup; 400g can chopped tomato; 450g sweet potato , peeled and cut into large chunks; few thyme sprigs; 1 bay leaf; handful parsley , chopped; Savoy cabbage , to serve