Cut sirloin into thinly-sliced into strips and season with salt and pepper.
In a large, hot, cast iron skillet, heat 2 tablespoons of butter over medium-high heat. Cook the steak in batches until browned, about 2-3 minutes per side. Transfer to a plate. Repeat with remaining steak, adding more butter if needed.
Add additional 2 tablespoons of butter same skillet. Once melted, add onion and mushrooms. Cook until onion softens and mushrooms start to brown, about 5 minutes. Add garlic and cook, stirring, for 1 more minute.
Add the flour and cook, stirring constantly, for about 1 minute. Gradually pour in the beef broth, stirring to ensure it's fully mixed. Add bouillon beef base. Sprinkle in ½ teaspoon of salt, and pepper. Simmer the mixture, stirring frequently, until it begins to thicken, approximately 5-8 minutes.
Incorporate the Worcestershire sauce, mushroom soup, sour cream, and Dijon mustard, stirring until the sauce is uniform. Allow the sauce to gently simmer until it reaches a thickness that coats the back of a wooden spoon. Finally, reintroduce the meat to the skillet and toss until it's thoroughly coated.
Mix with egg noodles and garnish with parsley.