Place the beef cubes into a large bowl and sprinkle with the all-purpose flour. Toss to coat the beef evenly.
Add the coated beef to the slow cooker.
Add the chopped onion, minced garlic, and sliced mushrooms on top of the beef.
Pour the beef broth over the ingredients in the slow cooker.
Add Worcestershire sauce, Dijon mustard, smoked paprika, salt, and black pepper. Stir gently to combine.
Cover the slow cooker and cook on low for 6 hours, or until the beef is tender.
About 20 minutes before serving, cook the egg noodles according to package instructions until al dente. Drain and set aside.
Stir the unsalted butter and sour cream into the slow cooker until fully incorporated and creamy.
Add the cooked egg noodles to the slow cooker and gently mix to combine with the beef and sauce.
Sprinkle chopped fresh parsley over the stroganoff before serving.