Place beef, mushrooms, onion, mustard, parsley, garlic, salt, dill and black pepper into a slow cooker; stir well.
Place flour in a small bowl; gradually add stock, stirring with a whisk until blended. Add to slow cooker; stir well. Cover with the lid; cook on High for 1 hour. Reduce heat to Low and cook until beef is tender, 7 to 8 hours.
Turn slow cooker off and remove lid. Allow stroganoff to stand for 10 minutes. Stir in soured cream. Serve over pasta.