Combine the beef, salt, vinegar, fish sauce, sesame oil, and ¼ teaspoon ground white pepper in a bowl. Mix well.
Bring the broth and mushrooms to a boil over high heat in a large saucepan. Lower the heat to maintain a simmer.
In a small bowl, whisk ¼ cup arrowroot powder with ¼ cup water to make a slurry.
Stir the arrowroot slurry into the simmering broth to thicken it.
Once the soup thickens, add the marinated meat and stir well. As soon as the meat is cooked through, turn off the heat. Taste the soup and season with salt and white pepper if needed.
In a slow, steady stream, pour in the beaten eggs whites from high above the pot, stirring as the whites hit the liquid. The whites will cook upon contact with the hot soup, forming ribbon-like tendrils.
Mix in the cilantro and scallions and stir well. Ladle the West Lake Beef Soup into bowls and serve immediately.