Trim fillet of excess fat. Season well and sear in a hot pan until all sides are nicely browned (about 12-15 minutes) allow to cool on a plate then smother with Dijon mustard and place in a fridge to chill. Finely dice onion and mushrooms and add thyme, for speed this can be done by pulsing them in a food processor. Melt butter in a pan over medium heat, add mushroom mix, cook for a few minutes then add breadcrumb and wine, cook until all liquid has evaporated. Adjust seasoning, allow to cool and then place in fridge. Place a large piece of clingfilm on a chopping board, lay prosciutto on the clingfilm slightly overlapping each slice. Spread half mushroom mix over the prosciutto, sit the fillet on top and spread remaining mix over. Using the film, roll the prosciutto over the fillet and roll into a sausage shape twisting the ends of the film as you go to tighten it. Chill again whilst sorting out pastry. Dust counter with flour and lay pastry on top. Unravel beef fillet from clingfilm and place on bottom end of pastry, brush other three edges with egg wash then tightly roll pastry over beef. Trim any excess and crimp edges to seal well. Cover in cling film again and chill for another ½ hour. Remove pastry and place roll onto a foil lined baking tray. Brush with egg wash and sprinkle with flaky salt. Add decorations with toothpick if required. Bake in oven set at 200C for around 35-45 minutes. Let rest for 10 minutes before serving.