Cook egg noodles to al dente per directions on the package. Drain and set aside.
Season the sliced beef with salt, black pepper, and thyme.
In a large skillet, heat two tablespoons of butter over medium-high heat. Place beef strips in a single layer, cooking for 1-2 minutes per side without stirring. Ensure they are just browned and no longer red. Sear the beef in two batches to avoid overcrowding the pan. Once browned, transfer the beef to a plate and cover to keep it warm.
Reduce heat to medium, add remaining butter, and melt. Then add onion and saute until tender, about 5 minutes. Add the mushrooms and garlic to the pan, and continue cooking until they have softened, about 4 minutes.
Add beef broth, flour, and Worcestershire sauce in a medium bowl. Whisk until there are no lumps, and pour into mushrooms and onions.
Bring to a boil, reduce the heat to low, and let simmer for 5 minutes.
Add the cooked beef back to the skillet when the mushroom gravy thickens, then stir in sour cream and Dijon mustard! Cook for 1-2 minutes until beef is warmed through.
Serve over cooked egg noodles, and enjoy!