In a large saucepan, heat oil over medium-high heat. Add onion,carrot and celery then cook, stirring, until soft. Add lamb mince and break up any lumps. Cook until lamb changes colour.
Add the flour and cook until combined. Add in stock,bay leaf,Worcestershire sauce and tomato paste. Bring mixture to the boil. Once boiling, reduce heat to low and cook, stirring occasionally, for 30 minutes. Taste and season with salt and pepper.
While the mince is simmering, cook potato in a saucepan of salted boiling water until tender. Drain well, then return to the pan and add 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.
Preheat oven to 200 degrees Celsius. Spoon lamb mince mixture into a 2L capacity ovenproof baking dish, or use four individual ovenproof serving dishes. Top with mashed potato. Brush with melted butter.
Bake in preheated oven for 20 minutes or until mashed potato is golden brown.
