Trim excess fat from meat and season generously with salt and pepper. (TIP: We buy a whole beef tenderloin that we will cut into filets. Our butcher asks how we would like the leftovers, and we ask for stew meat. This is the beef I use for stews like this. It actually turns out to be really economical and you get beef that is really tender!)
Heat butter in a large pan and add the beef, browning on all sides.
Turn heat down and push the beef to one side of the pan.
Add spring onions and cook slowly until soft.
Sprinkle flour over the meat and onions, stirring to coat and making sure flour is completely absorbed.
Pour in beef stock and bring to a boil, stirring constantly.
Lower the heat and stir in mustard.
Cover and simmer for 1 hour or until meat is tender.
10 minutes before serving, stir in mushrooms, sour cream, and white wine.
Taste to see if you need to season more. Serve.