Place the chicken, onion, brown mushrooms, cup mushrooms, stock, combined paprika, tomato paste, Worcestershire sauce, mustard and sour cream in a slow cooker. Season. Cover and cook for 4 hours on high (or 6 hours on low) or until the chicken is very tender.
Combine the cornflour with 2 tbs cold water in a small bowl. Add to the chicken mixture in the slow cooker and stir to combine. Cook for a further 5 mins or until the mixture thickens.
Meanwhile, cook the pasta following packet directions or until al dente. Drain well.
Add the butter to a large frying pan over high heat. Cook for 2 mins or until the butter melts and turns a nutty brown. Add the pasta and half the parsley and cook, tossing, for 1 min or until well combined.
Divide the pasta among serving bowls. Top with the chicken mixture. Season. Sprinkle with the remaining parsley to serve.