Place beef and 1 tbs paprika in a bowl. Season and toss to coat. Heat oil in a large, deep non-stick frying pan over high heat. Cook beef, in 2 batches, for 5 minutes, turning occasionally, or until well browned. Transfer to a 5.5L slow cooker.
Add onion, garlic, stock, wine, sauce, mustard and remaining paprika to pan and bring to the boil over high heat. Add to slow cooker, stirring to combine. Cook, covered, on low for 8 hours (or on high for 4 hours) or until beef is tender and soft enough to shred.
Using tongs, transfer beef to a heatproof bowl. Using 2 forks, coarsely shred beef. Return beef to sauce, then add mushroom. Stir to combine. Cook, uncovered, on high for 15 minutes or until sauce has thickened slightly and mushrooms are cooked.
Meanwhile, cook pasta in a large saucepan of boiling water according to packet instructions. Drain. Add pasta and chopped parsley to stroganoff, stirring to combine. Divide among plates. To serve, dollop over sour cream and scatter over extra parsley.