Heat 1 tablespoon of the beef dripping in a large cast iron pot set over a medium heat. Brown the mince (you may need to do this in batches) and when golden, remove from the pot with a slotted spoon and set aside.
Add the remaining tablespoon of beef dripping and turn the heat down to low. Add the chopped carrot, onion and celery along with 1 teaspoon salt and saute for 10 - 15 minutes, until the vegetables are completely tender, but not coloured. Add the herbs and flour and stir then add the wine and reduce by ½. Add the stock, Worcestershire sauce and tomato puree and stir to combine everything. Bring to a boil then turn down to a simmer and leave to bubble away for as long as possible - up to 2 hours, stirring occasionally. Pour into an ovenproof dish and leave to cool.
For the mash, cook the potatoes until soft in boiling salted water then drain and steam. Add the butter and double cream to a clean pan and set over a low heat. Pass the potatoes through a ricer into the pan and mix everything together. Add the parmesan and season well then add a good grating of nutmeg and place into a piping bag fitted with a large star nozzle. Pipe the mash onto the top of the pie and cook until the potato is golden and the sauce bubbling and piping hot - about 30 minutes.