Classic shepherd's pie

Name: Classic shepherd's pie
Prep time: 10 min
Cook time: 1 h 5 min
Total time: 1 h 15 min
Cuisine: British
Ingredients: 1 tbsp olive oil; 1 brown onion, halved, finely chopped; 1 carrot, peeled, finely chopped; 2 celery sticks, trimmed, finely chopped; 1 tbsp tomato paste; 500g lamb mince; Salt & freshly ground black pepper; 2 tbsp plain flour; 500ml (2 cups) Massel beef style stock; 1 tbsp worcestershire sauce; 1 bay leaf; 4 (about 200g each) desiree potatoes, peeled, chopped; 40g butter; 125ml (1/2 cup) milk; Melted butter, to brush