Preheat the oven to 190C/gas 5.
Lay the pork fillet out on a clean work surface and carefully cut into the centre, taking care not to cut straight the rough to the other side – the idea is to flatten the fillet out so that you can stuff it and roll it up. If the fillet is still a little thick in places, give it a gentle bashing with a rolling pin to even it out. Set it aside.
Melt the butter in a frying pan set over a medium heat and add the leek. Cook for 3 to 4 minutes, until softened but not coloured.
Add the apple and honey, turn up the heat slightly and cook for a further 5 minutes, or until the apple is softened.
Season with a little salt and black pepper, add the chopped walnuts and stir well.
Set aside until cool. Add the crumbled cheese and fold lightly into the mixture.
Season the inside of the pork fillet with a little salt and freshly ground black pepper, and then spread over the mustard.
Spoon the stuffing mixture in an even layer down the centre of the fillet. Roll up firmly
Carefully wrap the bacon slices around the fillet, overlapping them slightly.
Transfer the fillet to a roasting tin, drizzle with a tablespoon of oil and cook in the oven for about 25 minutes, until pork is just cooked but still has a rosy glow at the centre around the filling. Cover and leave to rest in a warm place.
In the meantime, heat the remaining oil in a large frying pan or wok and add the sliced leeks.
Cook over a medium heat for 5 minutes or so, until softened but not coloured.
Add the potatoes and celeriac and stir.
Pour in the white wine. Cook for 2-3 minutes, and then add the stock.
Allow the mixture to bubble over the heat for about 20 minutes, until the potatoes and celeriac are almost cooked through.
Add the cream, and cook for another 10-15 minutes, until the vegetables are really soft and creamy.
Stir in the parsley (leaving a little for the garnish) and season to taste.
Cut the pork fillet into slices. Spoon the vegetables onto a pretty serving platter, top with the pork slices and garnish with the remaining parsley. Serve with a smile.