Prepare the Filling: Heat 1 Tbsp of oil in a large pan. Add the ground pork and cook until browned, breaking it up as it cooks. Remove to a plate with a slotted spoon.
Add a little more oil to the pan. Add the leek, carrots, celery, and fennel/sage/thyme. Cook gently for 8 minutes, stirring occasionally.
Return the pork to the pan. Add the apple and Dijon mustard. Season with salt and pepper. Add enough wate to just cover the mixture, and simmer until tender (about 40 minutes).
Prepare the Mash: Meanwhile, place the potatoes in a pot with salted water. Bring to the boil and cook for 12-15 minutes, until tender.
Drain, and let steam dry for a couple of minutes. Mash with the butter and milk .Season generously.
Preheat oven to 190°C/Gas 5/375℉
Transfer the pork filling to a baking dish. Spread the mashed potato over the top.
Bake for 30 minutes, or until the top is lightly browned.