Prepare the broth: In a large pot, combine chicken stock and water. Add the onion, garlic, ginger, star anise, cinnamon stick, and fish sauce. Bring to a gentle simmer over medium heat.
Cook the noodles: While the broth is simmering, prepare the rice noodles according to package instructions. Drain and set aside.
Add the chicken: Once the broth has simmered for about 15 minutes, add the sliced chicken to the pot. Cook for an additional 5–10 minutes, or until the chicken is cooked through and tender.
Strain the broth: If desired, strain the broth to remove the solids for a clearer soup. Return the broth to the pot and adjust seasoning if needed.
Assemble the bowls: Divide the cooked rice noodles among serving bowls. Ladle the hot broth and chicken over the noodles.
Garnish & serve: Top with fresh herbs, green onions, bean sprouts, lime wedges, and a drizzle of Sriracha or hoisin sauce if desired.
Notes
Use fresh herbs for the best flavor and taste as you go to adjust the seasoning.