Heat the oil in a large frypan over medium heat. Add the beef and cook until browned. Add the mushrooms, capsicum and onion; cook and stir until starting to become tender, about 5 minutes.
In a slow cooker, combine the beef stock, salt, pepper, Worcestershire sauce, chilli flakes and red wine. Transfer the beef and vegetables to the slow cooke, and stir to blend. Cover and cook on High for 3 to 4 hours, until beef is extremely tender.
Preheat the oven to 220 degrees C. Drain the liquid from the slow cooker and save for dipping. Slice the French bread loaf lengthwise like a submarine sandwich. Mix together the horseradish and mustard; spread onto the inside of the loaf. Place slices of provolone cheese on both sides of the loaf, then fill with the beef and vegetables. Close the loaf and wrap the entire sandwich with aluminium foil.
Bake for 10 to 15 minutes in the preheated oven. For crunchier bread, you can bake it without the aluminium foil. Slice into servings, and serve with the juices from the slow cooker for dipping.