Nonstick spray is used to grease the bottom of a slow cooker 6-quart size or larger. Sprinkle the sirloin with salt, pepper, black pepper, garlic powder and onion powder. Mix to coat the meat.
Mix the mushrooms, 1 cup beef broth and Worcestershire with the Dijon. Cook on high for 2-3 hours or low for 5-7 hours until the beef is tender. If you prefer firmer mushrooms wait until halfway through the cooking time.
To make a slurry, combine the cornstarch with 1/2 cup of broth. The mixture should be incorporated into the slow cooker. Cook on high for 20 minutes. Stir in the Greek yogurt. Cover and continue cooking on high for 10 more minutes.
Cook the noodles according the package instructions while the stroganoff is cooking. Mix the noodles in the slow cooker, or place them in individual bowls. Pour the stroganoff on top. Serve immediately and garnish with fresh parsley or thyme if desired.