Classic shepherd's pie recipe

1 tbsp olive oil
1 brown onion, halved, finely chopped
1 carrot, peeled, finely chopped
2 celery sticks, trimmed, finely chopped
500g lamb mince
2 tbsp plain flour
1 tbsp Coles Tomato Paste
1 tbsp worcestershire sauce
1 bay leaf
500ml (2 cups) Massel beef style stock
4 (about 200g each) desiree potatoes, peeled, chopped
40g butter
125ml (1/2 cup) milk
Melted butter, to brush