Place the beef strips in the slow cooker.
Add the chopped onion, minced garlic, and sliced mushrooms on top of the beef.
In a medium bowl, whisk together the beef broth, Worcestershire sauce, Dijon mustard, paprika, salt, and black pepper.
Pour the broth mixture over the beef and vegetables in the slow cooker.
Cover and cook on low for 6 hours, or until the beef is tender.
About 30 minutes before serving, cook the egg noodles according to package instructions until al dente. Drain and set aside.
In a small bowl, mix the sour cream and flour until smooth.
Remove half of the cooking liquid from the slow cooker and whisk the sour cream-flour mixture into it until combined.
Return the mixture to the slow cooker and stir to combine. Add the butter and stir until melted and the sauce is creamy.
Add the cooked egg noodles to the slow cooker and gently toss to coat with the sauce.
Sprinkle chopped fresh parsley over the top before serving.