Shepherds pie with lambs liver

Name: Shepherd’s pie with lamb’s liver
Ingredients: 2 tbsp olive oil; 500g pack lamb mince; 250g lamb's liver (or chicken livers); 500ml chicken or lamb stock; 2 onions, finely chopped; 2 carrots, finely chopped; 2 celery sticks, finely chopped; a few thyme sprigs; 1 bay leaf; 3 tbsp tomato purée; 2 tbsp Worcestershire sauce, plus extra to serve; seasonal vegetables, to serve; 1 ¼kg Maris Piper potatoes, cut into large chunks; 150-200ml milk; 50g butter; 50g tangy cheese (such as mature Red Leicester), grated; 20g pack parsley, finely chopped