Slice the steak into thin strips and season well with salt and pepper.
Heat 1 tbs of the olive oil in a frying pan over high heat. Add half of the steak strips, spread out with tongs and allow to cook (without turning) for 30 seconds. Turn the steak and repeat for a further 30 seconds (the meat will still be pink inside). Transfer to a slow cooker and repeat cooking with the remaining olive oil and beef strips.
Add the butter to the frying pan and allow to melt. Cook the onion for 2-3 minutes or until soft. Add to the slow cooker.
In a bowl, mix together the beef stock liquid, dijon mustard, Worcestershire sauce, sweet paprika, dried thyme and tomato paste. Pour into the slow cooker and stir.
Cook for 8 hours on low or 5 hours on high.
With 1 hour of cooking time remaining, add the mushrooms.
With 30 minutes of cooking time remaining, stir through the sour cream.See notes - if you want to thicken the sauce. Serve with cooked egg noodles, pasta, rice or mashed potato.