Season the beef with salt and pepper.
Heat olive oil in a skillet over medium-high heat and brown the beef in batches.
Transfer the browned beef to the slow cooker.
Add mushrooms, onion, garlic, Dijon mustard, Worcestershire sauce, 1 1/2 cups of beef broth, and garlic powder to the slow cooker. Stir to combine.
Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the beef is tender.
In a small bowl, whisk together the remaining 1/2 cup of beef broth and cornstarch to create a slurry.
Stir the slurry into the slow cooker. Cover and cook on high for an additional 20 minutes, or until the sauce has thickened.
Meanwhile, cook the egg noodles according to package instructions.
Once the sauce has thickened, stir in the sour cream until fully incorporated.
Serve the beef stroganoff over the cooked egg noodles, garnished with chopped parsley if desired.
Enjoy!