Preheat the oven to 200C/400F/Gas 6.
For the steak Wellington, place the butter and flour in a food processor and process until the mixture resembles breadcrumbs. Add half of the beaten egg, the parsley and enough water to bind the mixture. Pulse until the mixture comes together to form a ball. Wrap the pastry in cling film and chill in the fridge while you prepare the beef.
Heat the oil in a frying pan, season the steak with salt and freshly ground black pepper and quickly sear the meat on all sides.
Roll the pastry out on a floured surface until large enough to enclose the meat. Place the seared steak into the middle of the pastry. Lay the spinach leaves over the top of the meat, followed by the cheese. Fold the pastry over and seal the edges by brushing with the remaining beaten egg. Brush the pastry all over with the remaining beaten egg and sprinkle over the sesame seeds.
Place onto a baking sheet, transfer to the oven and bake for 12-15 minutes, or until the pastry is golden-brown.
For the red wine jus, heat the red wine in a saucepan for about 4-5 minutes, until slightly reduced. Crumble in the stock cube and stir in the butter until melted.
To serve, cut the Wellington into slices and place onto a serving plate. Drizzle over the red wine jus.