Submerge dried bean thread noodles in warm water. Let soak for 10-15 minutes until softened.
Heat oil in a stock pot or large pot over medium-high heat. Once warm, add garlic and ginger. Stir often and cook until fragrant, about 30-45 seconds. Add onions and stir to combine.
Once onions are almost translucent, about 2 minutes, add raw chicken. (Skip if using cooked chicken.) Cook until chicken registers at least 165 degrees F. Turn chicken as needed to cook evenly, about 5 minutes.
Add mushrooms and cook for 2 minutes. Stir as needed to ensure onions do not burn at the bottom of the pot.
Add chicken broth. Remove cooked chicken and place on plate or cutting board to cool. Bring broth to a boil. Let boil for about 2 minutes. Reduce heat to a simmer.
Shred cooled chicken using two forks. Add shredded chicken to stock pot. Season broth with salt. Taste. Add more salt, as needed.
Drain soaked noodles. Add to pot. Stir to distribute. Bring soup back to a boil. Let boil for 2 minutes. Reduce heat to a simmer and cook for another 3-5 minutes.
Ladle soup and noodles into serving bowls. Season with black pepper or white pepper according to taste. Garnish with chopped green onions.