Heat a drizzle of oil in a heavy skillet set over medium-high heat.
Sprinkle the steak with salt and pepper, thinly slice it across the grain, and cook for 2-3 minutes, turning with tongs, until the strips are just cooked through.
Transfer to a plate and add the butter to the pan.
Cook the onion and mushrooms for 4-5 minutes, until soft and starting to brown on the edges.
Sprinkle with flour, stirring to coat, and stir in the stock and Worcestershire sauce. Bring to a simmer, stirring until the mixture thickens.
Stir in the sour cream and add a splash of water if it seems too thick.
Return the beef to the pan and serve over egg noodles.