Heat the oil in a large saute pan, add the onion and cook for 2-3 minutes. Add the carrots and mushrooms and continue cooking for 5 minutes until the vegetables start to colour.
Meanwhile prepare the potato and parsnip topping. Place the potatoes and parsnips in a large pan of salted water, bring to the boil and simmer for approximately 15 minutes until tender. Drain and mash with the butter and milk. Beat in the leek if using.
If using cooked minced lamb - add the stock, ketchup, Worcester sauce and thyme to the pan and mix together. Gently fold the cooked lamb through the mixture and gently heat through. Season with black pepper and salt if required.
If using uncooked lamb mince - add the lamb to the saute pan with the vegetables and brown, stirring to break up any lumps. Sprinkle on the flour and mix through. Cook for 1-2 mins, stirring to make sure the mixture does not stick to the bottom of the pan.
Add the stock, tomato ketchup, Worcester sauce, black pepper and cook over a low heat, simmering for approx 45 minutes. Check seasoning and adjust if required.
Add the meat to a medium sized ovenproof dish and top with the potato mixture, using a fork to spread and make ridges on top.
place in the oven and bake for about 25 - 30 minutes until piping hot and the potato is golden.