Season the beef with the salt and freshly ground black pepper and sprinkle with the thyme.
Heat a large frying pan until very hot, then drizzle with the oil. Add the beef and cook briefly, turning occasionally, until golden-brown on all sides.
Remove the beef and allow to cool.
Spread the mustard all over the beef, then spread the pâté evenly over one side of the beef.
Roll the puff pastry out to dimensions that will wrap around the beef to completely enclose it.
Place the beef on top of the pastry and roll it up to form a parcel. Brush the edges with beaten egg and press down to seal.
Trim away any excess overlapping pastry and brush the beef Wellington all over with beaten egg. Transfer to the fridge to chill for 30 minutes.
Preheat the oven to 200C/400F/Gas 6. Place a baking tray in the oven to preheat.
Place the beef Wellington onto a sheet of buttered baking parchment, then transfer onto the hot baking tray (so that the beef Wellington cooks evenly).
Transfer to the oven to bake for 30 minutes, until the pastry is golden-brown and the beef is cooked to your liking. (A meat thermometer is a good guide in this case.)
Remove from the oven and allow to rest for five minutes, then serve in slices with your chosen vegetables.