Bring the potatoes to the boil and leave to simmer for 20 minutes. Once they’re tender drain and mash. Add a little butter and some milk and mix well, until the desired texture is reached. Season as desired.
Whilst boiling the potatoes, heat a little oil in a frying pan and fry the onions and mushrooms for around 10 minutes until softened. Add the lamb mince and fry until it’s browned all over.
Stir in the tomato puree, ketchup, Worcestershire sauce and mustard powder. Add the stock and season as desired. Leave to simmer for around 30 minutes.
Preheat the oven to 200 C. Spoon the mince mixture into an ovenproof dish and cover with the mashed potato. Sprinkle the cheese over the top. Place in the preheated oven for 25-30 minutes until golden brown.