Char the onion and ginger over an open flame to create a char on the outside. Let cool and peel the outside of the ginger with a spoon.
Add two quarts of water to a large pot and bring to a simmer. Add the bouillon, mushroom seasoning, and the charred vegetables. Then add scallions, fish sauce, and rock sugar and stir to dissolve. Bring the a boil.
Toast the spices in a dry pan until fragrant. Should take about 3 minutes. Put in a cheesecloth and tie off. Then you can add the spice bag directly in the pot.
Lower the heat and let simmer for at least thirty minutes to 1 hour. This is enough times for the favors to develop.
In the meantime, boil the noodles until just done. Rinse with cold water to maintain the QQ (chewiness)
Prepare any other garnishes you would like on top.
Taste the broth and adjust taste to your preference. Remove the spice bag and strain the broth.
To serve, add noodles to a bowl. Arrange garnishes on top and pour boiling hot broth on top. Enjoy hot!