In a large skillet over medium-high heat, brown the beef pieces in batches, seasoning with salt and pepper. Transfer the browned beef to the slow cooker.
In the same skillet, add the chopped onion and sauté for about 3 minutes, until softened. Add the minced garlic and sauté for an additional minute until fragrant.
Pour the beef broth into the skillet to deglaze, scraping up any browned bits from the bottom. Stir in Worcestershire sauce, Dijon mustard, paprika, and add mushrooms. Pour this mixture over the beef in the slow cooker.
Cover and cook on low for 6 hours, or until the beef is tender.
In a small bowl, mix the sour cream with flour until smooth. Stir this mixture into the slow cooker during the last 30 minutes of cooking.
Meanwhile, prepare the egg noodles according to the package instructions. Drain and set aside.
When the beef is ready, serve the creamy beef stroganoff over the cooked egg noodles. Garnish with fresh parsley if desired.