Char the onion halves and ginger under a broiler or in a dry skillet until deeply browned.
Toast coriander, fennel, cloves, star anise, and cinnamon in a large pot over medium heat until fragrant.
Pour in chicken broth, then add chicken thighs, charred onion, and ginger. Bring to a gentle simmer.
Skim any foam. Simmer uncovered until the chicken is cooked through, about 25–30 minutes.
Lift out the chicken. When cool enough, discard skin and bones, then shred the meat.
Strain the broth to remove spices and aromatics. Return clear broth to the pot and season with fish sauce, brown sugar, and salt.
Soak rice noodles in hot water until just tender, 8–12 minutes. Drain well.
Divide noodles and shredded chicken among bowls. Ladle in steaming broth.
Top with white onion, green onions, cilantro, basil, and sprouts. Serve with jalapeño, lime, hoisin, and sriracha.