Pound your ribeye steak so it's nice and thin and slice into thin (1/4-inch) strips, removing any excess fat. Sprinkle the strips with salt and pepper and toss to coat.
Add the olive oil to a large skillet over high heat, add in the steak and perform a quick sear, flip the steak and sear the other side just until brown, remove from pan. *You might have to cook the beef in two batches.
Add the butter, diced onions, chopped mushrooms to the pan and cook over medium-high heat until for 2 to 3 minutes until onions are tender.
Sprinkle the flour into the pan and stir for 1 minute, slowly add in the beef broth while continuing to stir. Make sure to scrape the bottom of the pan to deglaze it and get all those bit of flavor off the bottom.
Stir in the sour cream, garlic powder, and mustard until fully combined, reduce heat to low and allow the sauce to simmer for 2 to 3 minutes.
Add the steak and any juices back into the sauce and simmer for another 1 to 2 minutes to heat the meat through. Add additional salt and pepper to taste.
Serve over or tossed with egg noodles.