Sear beef over high heat in a little oil until well browned
Add onion, mushrooms, garlic and paprika, fry for a few minutes
Add flour and mix well until all flour is soaked up
Mix in mustard, Worcestershire sauce, hot water and stock pots
Mix well
Transfer to slow cooker and cook on low for 7-8 hours or high for 3-4 hours
Mix in sour cream and serve
