01 - Place chicken, onion, ginger, and water in a large stockpot. Bring to a rolling boil over high heat, then reduce to medium-low heat. Simmer for 30 minutes, skimming off any foam or impurities that rise to the surface. Remove the chicken and set aside to cool.
02 - Continue simmering the broth with onion and ginger for 45 minutes longer. Add salt, fish sauce, and rock sugar. Strain the broth through a fine mesh strainer for clarity and return to the pot. Keep warm over low heat.
03 - Shred the cooled chicken breast into thin, uniform strips. Reserve 2 cups for serving. Store remaining meat for another use.
04 - Whisk eggs with 1/4 tsp salt until well combined. Heat a nonstick skillet over medium-low heat. Pour a thin layer of egg mixture to create a flat crepe. Cook until just set, about 1 minute, then flip briefly and remove. Repeat with remaining egg mixture. Slice crepes into fine julienne strips.
05 - Prepare rice vermicelli according to package instructions (typically boil 2-3 minutes until tender). Rinse thoroughly under cold water to stop cooking and drain well.
06 - Slice Vietnamese pork sausage into thin julienne strips, approximately 2 inches long.
07 - Divide cooked noodles among 4 serving bowls. Arrange shredded chicken, egg crepe strips, and pork sausage on top. Garnish with scallions, cilantro, Vietnamese coriander, mint, and sliced chili.
08 - Ladle the hot, clear broth over the toppings just before serving. Top with dried shrimp floss if using. Serve with lime wedges and shrimp paste on the side.