Soak the shiitake mushroom and black fungus in just-boiled water for at least 20 minutes. Once done, rinse under cold water and cut into thin slices.
Add the chicken stock and water into a pot on medium high and bring to the boil.
Meanwhile, put the onion and ginger in a fry pan on medium-high heat. Dry fry them until they are charred, then add them to the stock.
Once the pot is boiling, turn down the heat to medium-low and add the shiitake mushrooms. Partially cover the pot with the lid and allow to simmer for 20 mins.
In a small pot, add boiling water and cook the black fungus for 5 - 6 minutes. Once done, strain and set aside.
Prepare the glass noodles as per packaging.
After 20 minutes, add the chicken breast to the pot and cook for roughly 8 -10 minutes or until done. Once the chicken is fully cooked, cut or tear into pieces.
Add the fish sauce, sugar and pepper to the soup. Check the seasoning and adjust to your taste.
Turn the heat up to medium-high and allow the soup to come to a rolling boil.
Divide the noodles into bowls and pour boiling water into them. Strain the noodles and add the chicken, black fungus and bean sprouts (if using) on top.
Ladle the boiling soup into the bowls and garnish with chopped spring onion and fried shallots.
Serve with lemon wedges and chilli sauce (optional).