• Gather all ingredients and have your skillet and cutting board ready.

For the Beef:

• Pat the steak dry, remove any remaining fat, then slice very thinly across the grain into roughly 1/5-inch strips; cut long pieces in half. Sprinkle with the salt.
• Heat the olive oil and 1 tablespoon butter in a large skillet over medium-high. In batches to avoid crowding, sear the beef strips in a single layer about 30 seconds per side until browned; transfer the browned strips to a plate and reserve any juices.

For the Sauce:

• Lower the heat to medium and add 2 tablespoons butter to the same skillet. Sauté the diced onion until it softens and turns translucent, about 3 minutes.
• Add the mushrooms and cook until they begin to soften and release liquid, about 3 minutes. Stir in the minced garlic and cook another 30 seconds until aromatic.
• Sprinkle the flour over the mushroom mixture, stir well, and cook briefly for about 30 seconds to remove the raw flour taste.
• Pour in the beef broth, scraping the pan to release browned bits. Bring to a simmer, reduce heat to medium, and let the sauce thicken for about 5 minutes while stirring occasionally.
• Whisk in the Worcestershire sauce (if using), Dijon mustard, and sour cream until the sauce is smooth. Keep the heat low so the sour cream blends without curdling.

Finish:

• Return the seared beef and any juices to the skillet, toss to coat in the sauce, and simmer for 1 minute to heat through. Remove from heat and serve over egg noodles or rice; garnish with chopped parsley or chives.