Jackie Smith's Classic Shepherd's Pie

Ingredients

• 1 tbs olive oil
• 1 brown onion, halved, finely chopped
• 1 carrot, peeled, finely chopped
• 2 celery sticks, trimmed, finely chopped
• 500g lamb mince
• 2 tbs plain flour
• 500ml (2 cups) beef stock
• 1 dried bay leaf
• 1 tbs Worcestershire sauce
• 1 tbs tomato paste
• Salt and freshly ground black pepper
• 4 (about 200g each) desiree potatoes, peeled, chopped
• 40g butter
• 125ml (½ cup) milk
• Melted butter, to brush