For the fennel and mushroom duxelles, blend the fennel, mushrooms and onion in a food processor to a paste.
Heat a pan over a low to medium heat, then add the butter. When the butter is foaming, add the fennel and mushroom paste and cook until all of the moisture has evaporated from the paste. Season, to taste, with salt and freshly ground black pepper.
Stir in the cream, then continue to cook the paste until heated through and well combined.
For the parsley pancakes, sift the flour into a large glass bowl. Add the sugar. Season, to taste, with salt and freshly ground black pepper.
Make a well in the centre of the flour and sugar mixture. Gradually add the milk and beaten egg, a little at a time, whisking well after each addition until all of the liquid has been incorporated into the batter before adding the next.
Cover the pancake batter with cling film, then chill in the fridge for 30 minutes.
When the pancake batter has chilled, stir in the melted butter and chopped parsley.
Heat one tablespoon of the oil in a frying pan over a high heat. When the oil is smoking, add a ladleful of the chilled pancake batter, then swirl the pan until it covers the bottom of the pan evenly. Fry for 30 seconds, then flip over and continue to cook for a further 30 seconds. Set aside on a plate and cover with a sheet of greaseproof paper. Keep warm.
Repeat the process until all of the pancake batter has been used up (you should get three pancakes from this mixture). Separate each pancake with greaseproof paper and keep the pancakes warm.
For the venison en croûte, line a baking tray with greaseproof paper.
Heat a frying pan over a medium to high heat, then add the butter. When the butter is foaming, fry the venison loin on all sides until browned all over. Remove the venison loins from the pan and set aside to rest on a plate.
Roll out the puff pastry onto a lightly floured work surface to a 25cm x 30cm/10in x 12in rectangle.
Lay the three pancakes on top of the pastry rectangle to cover the pastry. Spread three tablespoons of the duxelles mixture onto the pancakes.
Place the seared venison into the centre of the pancakes. Spread the remaining duxelles mixture all over the top of the venison loin.
Fold the pancakes as tightly as possible around the venison (do not allow the duxelles mixture to seep out).
Brush the edges of the pastry with some of the beaten egg, then wrap the pastry around the venison and pancakes, overlapping the edges and folding to seal. Brush the outside of the pastry with more of the beaten egg.
Transfer the venison parcel to the prepared baking tray, then chill in the fridge for 30 minutes.
Preheat the oven to 220C/425F/Gas 7.
When the venison parcel has chilled, brush all over again with the remaining beaten egg. Bake in the oven for 15-20 minutes, or until the pastry has risen and is golden-brown (the venison will be cooked to medium after 15-20 minutes). Remove from the oven and set aside to rest for five minutes.
For the buttered Savoy cabbage, blanch the cabbage leaves in a pan of boiling, salted water for 3-4 minutes, or until just tender. Drain well, then refresh in cold water. Drain again and squeeze the leaves dry. Slice the blanched leaves into strips.
Heat the butter in a frying pan over a medium heat. Add the blanched cabbage leaves and fry for 3-4 minutes, or until coated in the melted butter. Just before serving, stir in the sage until well combined.
While the venison is cooking, reduce the beef stock in a pan over a medium heat, until the volume of liquid has reduced to 100ml/3½fl oz. Keep warm.
To serve, slice the venison en croûte into four slices, each approximately 0.75cm/1½in thick. Place one slice into the centre of each serving plate. Serve the buttered cabbage alongside. Drizzle over the reduced beef stock.