Cook the noodles. Bring a large pot of water to a boil. Add the salt followed by the noodles. Cook per the package instructions to al dente. Turn off the heat, drain the noodles, and return them to the pot. Stir the butter into the noodles until melted.
Cook the steak. Heat the olive oil over medium-high heat in a large heavy-bottomed pan. Add the steak to the pan and season with salt and pepper. Saute until nicely caramelized and almost cooked through. Use a slotted spoon to transfer the steak to a plate. Set aside.
Saute the veggies. Reduce the heat to medium and add the butter to the pan. Add the red onion and saute until nicely caramelized. Add the mushrooms and season with salt, pepper, and garlic powder. Saute until the mushrooms have caramelized.
Make a roux. Add the flour to the mushrooms and cook for 1 minute, stirring consistently.
Deglaze. Add the white wine. Cook for 1-2 minutes, stirring occasionally.
Make it saucy. Add the broth, heavy cream, and Worcestershire sauce. Bring the mixture to a boil and then reduce the heat to low. Simmer for 5 minutes or until the sauce has thickened, stirring occasionally.
Put it all together. Stir the sour cream into the sauce. Stir in the beef. Let it all simmer for a minute or so until the beef is heated through. Turn off the heat.
Serve. Serve the sauce over the buttery noodles and garnish with fresh parsley.