Place the noodles in 2-1/2 quarts of already-boiling unsalted water and cook 5 minutes.

Meanwhile, if the beef is frozen, run it under hot water so can remove any packaging. Place the meat on a microwave-safe plate and microwave 2 minutes, uncovered, on high, to begin defrosting. While the meat microwaves, peel the onion, then begin heating a 12-inch nonstick skillet over medium heat.

Add the beef (fresh or partially defrosted) to the skillet, raise the heat to medium-high, and cook, stirring often, turning and breaking up the meat as it begins to brown. Coarsely chop the onion, adding it to the skillet as you chop. Add the garlic and mushrooms. Continue to cook, stirring frequently, until the meat is crumbled and browned, about 5 minutes. Add the green beans to the pasta pot, cover the pot to bring it back to a boil, then crack the lid, and cook until the noodles are tender and the beans heated through, 5 minutes more.

Meanwhile, add the sherry, ketchup, mustard, Worcestershire, paprika, and the mushroom soup to the skillet. Stir well until it is completely incorporated. Lower the heat to simmer the mixture, then add the sour cream and continue to cook, stirring constantly, until the sour cream is completely incorporated, 1 minute. Season with salt and pepper.

Drain the noodles and toss to evenly distribute the beans. Place some on each of the serving plates. Top with the sauce and serve.