Classic Shepherd's Pie

Mum gave me the idea today to make a shepherd's pie for dinner tonight.  I don't think I've ever made one before but I've always loved eating them...  This one is from taste.com.au and it was delicious - perfect for a freezing Melbourne evening.  I served ours with blanched spinach, I meant to add peas as well but didn't have any in the freezer.  It's a pretty large serve for 4 pax, we didn't get through half of it.

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
500 ml (2 cups) beef stock
1 dried bay leaf
1 tbsp Worcestershire sauce
1 tbsp tomato paste
Salt and freshly ground black pepper
4 (about 200g each) desiree potatoes, peeled, chopped
40g butter
125ml (1/2 cup) milk
Melted butter, to brush

Preheat oven to 200 degrees C.
Heat oil in a large saucepan over med-high heat.
Add onion, carrot and celery and cook stirring for 5 minutes or until soft.
Add lamb mince and cook, stirring to break up any lumps, for 5 minutes or until lamb changes colour.
Add the flour and cook, stirring for 2 minutes or until combined.
Add stock, bay leaf, Worcestershire sauce and tomato paste.
Bring to the boil.
Reduce heat to low and cook, stirring occasionally for 30 minutes or until sauce thickens.
Taste and season with salt and pepper.
Meanwhile, cook potato in a saucepan of salted boiling water for 15 minutes or until tender.
Drain well.
Return to the pan with the butter.
Use a potato masher or fork to mash until smooth.
Add milk and use a wooden spoon to stir until combined.
Taste and season with salt and pepper.
Spoon lamb mixture into a 2L (8 cup) capacity ovenproof baking dish.
Top with mashed potato and use a fork to spread over lamb mixture.
Brush with butter.
Bake in preheated oven for 20 minutes or until mashed potato is golden brown.
Serve immediately.
Serves 4.
Recipe by Sarah Hobbs, from taste.com.au / http://www.taste.com.au/recipes/2335/classic+shepherds+pie