01 - Place the whole chicken, onion, smashed ginger, and water in a large stockpot. Bring to a rolling boil over high heat, then reduce to a gentle simmer. Skim off any foam or impurities that rise to the surface for a clear broth. Simmer for 30 minutes until the chicken is cooked through.
02 - Carefully remove the chicken from the broth and transfer to a plate to cool completely. Continue simmering the broth with the onion and ginger for an additional 45 minutes to develop depth of flavor. Add salt, fish sauce, and rock sugar. Strain the broth through a fine mesh sieve for clarity and return to the pot to keep hot.
03 - Once the chicken is cool enough to handle, remove the skin and bones. Shred the breast meat into thin, uniform strips. Reserve 2 cups for serving and save the remaining meat for another use.
04 - Whisk the eggs with 1/4 teaspoon salt until well combined. Heat a nonstick skillet over medium-low heat and lightly grease. Pour a thin, even layer of egg mixture to create a delicate crepe. Cook until just set, about 30 seconds, then carefully flip and cook for another 10 seconds. Transfer to a cutting board. Repeat with remaining egg mixture. Stack the crepes and slice into fine julienne strips.
05 - Prepare the rice vermicelli noodles according to package instructions (typically boiling for 3-5 minutes until tender). Drain thoroughly and rinse under cold running water to stop the cooking process and prevent sticking. Shake off excess water and set aside.
06 - Slice the Vietnamese pork sausage (gio lua) into thin, uniform julienne strips, approximately 2 inches in length. Set aside in a small bowl until assembly.
07 - Divide the cooked rice vermicelli among 4 individual serving bowls. Arrange the shredded chicken breast, egg crepe strips, and pork sausage attractively on top of the noodles. Scatter the scallions, cilantro, Vietnamese coriander, mint leaves, and sliced chili over each bowl.
08 - Ladle the hot, clear broth directly over the arranged toppings in each bowl. The hot broth will slightly wilt the herbs and warm all components. Garnish with dried shrimp floss if using and serve with fresh lime wedges on the side. Accompany with shrimp paste (mam tom) for those who enjoy its distinctive pungent flavor.