01 - Place chicken, onion, ginger, and water in a large stockpot. Bring to a rolling boil over high heat, then reduce heat to maintain a gentle simmer. Skim off any foam or impurities that rise to the surface. After 30 minutes, carefully remove the chicken and set aside to cool.
02 - Continue simmering the broth with onion and ginger for an additional 45 minutes to develop depth. Add salt, fish sauce, and sugar. Strain the broth through a fine mesh strainer for clarity and return to the pot to keep hot.
03 - Once cooled, shred the chicken breast into thin strips using your fingers or two forks. Reserve the meat for serving and discard skin and bones.
04 - Whisk eggs with 1/4 teaspoon salt. Heat a nonstick skillet over medium-low heat. Pour a thin layer of egg mixture to create a flat crepe. Cook until just set, approximately 30 seconds, then flip briefly and remove. Repeat with remaining egg. Slice the cooked crepes into fine julienne strips.
05 - Prepare rice vermicelli according to package directions (typically boil for 3-5 minutes until tender). Drain thoroughly and rinse under cold running water to stop cooking and prevent sticking. Drain well before serving.
06 - Slice Vietnamese pork sausage into thin julienne strips, approximately 2 inches in length, to match the other toppings.
07 - Divide cooked noodles evenly among serving bowls. Arrange shredded chicken, egg crepe strips, pork sausage, scallions, cilantro, Vietnamese coriander, mint, and chili on top of the noodles.
08 - Ladle the hot, clear broth over the arranged toppings. Garnish with shrimp floss if using and serve with lime wedges and shrimp paste on the side for guests to adjust seasoning to their preference.