Cook the egg noodles - Bring a large pot of salted water to a boil. Add the egg noodles and cook according to package instructions, stirring occasionally. When they are tender, drain them in a colander and set them aside. - Make sure to salt the water generously to prevent the noodles from sticking together. You can keep the noodles warm by placing them in a bowl covered with a lid or aluminum foil.

Prepare the beef and onions - Slice the beef tenderloin into thin strips against the grain. Season with salt and black pepper. Peel and chop the onion, and mince the garlic. In a large skillet, heat the olive oil over medium-high heat. Add the beef strips and cook until browned on all sides, then remove them from the skillet and set them aside. In the same skillet, add the chopped onions and a pinch of salt, and cook until they become translucent, then add the minced garlic and cook for an additional 30 seconds. - Cutting the beef against the grain makes it tender and easier to chew. You can use a meat mallet or the back of a knife to gently pound the meat and make it even thinner.

Simmer the stroganoff - Add the cream of mushroom soup to the onions and garlic in the skillet. Stir well until the soup is heated through, then add the cooked beef strips and any juices that have accumulated on the plate. Bring the mixture to a simmer and cook for about 10 minutes, stirring occasionally. - If the stroganoff is too thick, you can add some beef broth or water to thin it out. If it's too thin, let it simmer for a few more minutes until it reduces to the desired consistency.

Stir in the sour cream and parsley - Remove the skillet from the heat and stir in the sour cream until it is well incorporated. Add the cooked egg noodles to the skillet and toss to coat them in the stroganoff. Sprinkle the parsley on top and serve. - Make sure the skillet is off the heat before adding the sour cream to prevent curdling. You can use fresh or dried parsley depending on what you have on hand.