Boil chicken in ~5 cups water (my mom's specific instruction was enough water to cover the chicken x 2) and 2 tablespoons of fish sauce until just underdone, about 3-5 minutes once water boils. Note: chicken does not need to be fully cooked, only shreddable. While chicken is cooking, skim away bits that float at the surface. Then remove chicken to cool for shredding and save broth.
While water is boiling, submerge dried mushroom in water. Mushrooms will soften in about 5-10 minutes.
While chicken cools slightly, chop vegetables:- Rough chop dark green part of 2 bulbs green onions for stir fry - Rough chop dark and light green parts of other 2 green onion bulbs along with Vietnamese coriander, mixing and saving for garnish- Thinly slice mushrooms- Give now softened dried mushrooms a few chops Shred chicken
Add shredded chicken to pan with 2 spoonfuls of broth (enough to keep moist), mushrooms, black pepper, 1 tablespoon fish sauce, green onions. Cook about 7 minutes or until mushrooms fully cooked. Set aside.
Add chicken broth to reserved chicken cooking liquid. Season with pepper and additional fish sauce (I usually add another 1-1.5 tablespoons).
Cook noodles directly in broth until translucent (3-4 minutes) just before serving. Layer bowls: pull out noodles first, chicken/mushroom, broth, Vietnamese coriander/green onion mix, fried shallot.