Add steak, onion, garlic, stock, mustard, paprika and worcestershire sauce to a slow cooker and season to taste with salt and pepper.
In a small bowl, whisk together cornflour and water thoroughly to combine into a paste and add to slow cooker. Stir to combine
Cover and cook on low heat for 6 hours. Remove lid and bring to the boil, uncovered. Add mushrooms. Allow to boil until most of the liquid has reduced.
Add sour cream and turn slow cooker off. Stir to combine and check for seasoning. Serve with mashed potato, pasta or rice.
Notes
If your slow cooker doesn't have a "Saute" setting that allows it to boil, then simply transfer to a large sauce pan!