Char the onion halves and ginger over an open flame or under a broiler until slightly blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes.
Add the chicken broth, charred onion, ginger, and fish sauce to the pot and bring to a gentle simmer.
Submerge the chicken breast in the broth and poach for 12-15 minutes until cooked through, then remove and shred.
While chicken poaches, cook the rice noodles according to package directions, drain, and set aside.
Strain the broth to remove the solids and return the clear liquid to the pot, seasoning with salt and pepper.
Divide the noodles and shredded chicken into a large bowl.
Ladle the hot broth over the noodles and top with sesame oil, bean sprouts, cilantro, sliced scallions, and a squeeze of lime.