Simmer parsnips in salted water for 15-20 minutes until tender.
Fry onions in butter over moderate heat until soft and translucent.
Stir in cumin, coriander, and garam masala and cook for 2 minutes.
Drain parsnips, mash with butter and seasoning, then stir in the spiced onions.
Sauté leeks and carrots in butter until meltingly soft and golden.
Add mushrooms for 5 minutes, then brown the lamb mince for another 5 minutes.
Stir in flour and tomato puré and cook for 4 minutes.
Pour in 250ml hot stock, add herbs, and bring to the boil.
Simmer the filling very gently for 25-30 minutes.
Preheat the oven to 190°C.
Season the filling generously with salt, pepper, and Worcestershire sauce.
Transfer filling to a baking dish, top with parsnip mash, and bake for 25 minutes.