Dry the beef with kitchen paper. Season with salt and pepper. Brown the beef on all sides. This helps to seal the meat so that it holds it shape and retains its juices. Overlap the prosciutto on a large piece of cling film. Spread with the stuffing, cover with cling film and roll to an even layer. Remove cling film and lay the beef on top of the mixture. Lift and roll the cling film to encase the beef. Roll the pastry to a rectangle and brush with beaten egg. The beaten egg will give the pastry an attractive golden color when cooked. Fold up the shorter edges, then roll up. Chill for around 30 mins. Brush with beaten egg to glaze the pastry. Score lightly to decorate. Sprinkle with sea salt and cook in a preheated oven. Leave to rest for around 10 mins then slice with a sharp knife.