Prepare the aromatics: Char the halved onion and ginger directly over an open flame on your stove or under a broiler until slightly blackened. This adds crucial smoky flavor.

Toast spices: In a dry pan, lightly toast star anise, cinnamon stick, cloves, cardamom, and coriander seeds until fragrant, about 1-2 minutes. Be careful not to burn.

Make the broth: In a large stockpot, combine the chicken, charred onion, ginger, toasted spices, water, fish sauce, sugar, and salt. Bring to a boil, then immediately reduce heat to a gentle simmer. Skim off any foam or impurities that rise to the surface for a clear broth. Continue to simmer for at least 2-3 hours, or up to 4 hours, until the chicken is tender and the broth is deeply flavorful.

Remove chicken and debone: Carefully remove the chicken from the broth. Once cool enough to handle, remove the meat from the bones and shred or slice into bite-sized pieces. Set aside. Return bones to the pot and continue simmering the broth for another hour if desired for more flavor.

Strain the broth: Strain the broth through a fine-mesh sieve into a clean pot, discarding the solids. Taste and adjust seasoning with more fish sauce, salt, or sugar if needed. Keep the broth warm.

Prepare noodles: Cook the rice noodles according to package instructions. Fresh noodles usually just need to be blanched quickly in boiling water, while dried noodles need to be soaked and then boiled. Drain well.

Assemble Pho: Divide the cooked noodles among serving bowls. Top with shredded chicken. Ladle hot broth over the noodles and chicken. Garnish with fresh bean sprouts, Thai basil, cilantro, green onions, and a lime wedge. Serve immediately with optional sliced jalapeño.