Trim your steak from any fat, and slice it thinly into ⅕ inch against the grain. Cut any long strips in half. Season with salt.
In a large skillet over medium-high heat, heat olive oil and melt the butter. Sear the beef strips in a single layer in batches, 30 seconds per side, then remove onto a plate and set aside.
In the same skillet, melt butter and cook onion until soft and translucent, about 3 minutes.
Add mushrooms, and cook for 3 more minutes or until they start to soften. Add garlic, and saute for 30 seconds or until fragrant.
Sprinkle flour all over the mushrooms, stir, and cook for 30 seconds.
Pour the beef broth into the pan, deglaze it, and scrape off any bits stuck to the bottom of the pan. Stir, lower the heat to medium, and allow the mixture to simmer and thicken for 5 minutes.
Add Worcestershire sauce, Dijon mustard, and sour cream to the sauce. And stir until well incorporated.
Add sauteed beef strips to the sauce with any juices left on the plate, stir to combine, and simmer for 1 minute. Remove from heat and serve over egg noodles, garnish with chopped fresh parsley or chives.