Place cube of beef stock in 1/4c boiling water and leave to dissolve while you prepare other ingredients 

Slice mushrooms, onion and steak into thin strips

In a very hot pan, quickly sear steak with oil. This should only take a couple of seconds each side. The inside and some of the outsides should remain pink (see picture above). This is not to cook the steak, only to sear it (it will cook later on). Remove steak from pan immediately and place in a bowl to rest

In the same (now empty) pan, turn down the heat a little and add onion and garlic and cook until soft.

Add mushrooms and continue cooking 

Pour sour cream, beef stock and mustard into pan and stir in. Sprinkle the flour on top and stir this in too.  

Once this is all mixed and boiling, add steak strips, brandy and wine and cook for 3-5 minutes until steak is cooked through 

If the sauce is too thin, you can add some more flour. If it is too thick, dilute with some more wine or water.

Serve with pasta of your choice