Place the chicken in a pot and cover with water. Season liberally with salt and add the sliced onion and halved jalapeño. Bring to a boil and then reduce heat and simmer for at least an hour. Skim off any foam as it floats to the surface.
Shred the chicken:
Remove chicken from the broth and debone it once it’s cool enough to handle. Shred the chicken meat and reserve the carcass for another use.
Strain the broth:
Pour the broth through a sieve into a large bowl. Wipe out the pot and return the broth to the pot. Taste and season.
Finish the broth:
Return the broth back to a boil and add the shredded chicken, shichimi togarashi, and chili or sesame oil. Add the minced white parts of the scallions. Simmer for an additional 30 minutes until the broth is dark and rich. Taste and add more salt or shichimi togarashi, if needed.
Cook the noodles:
As the broth finishes cooking, bring a pot of water to a boil. Cook the soba according to package instructions. Drain, rinse, and divide between bowls.
To serve:
Ladle the chicken and broth over the cooked soba and garnish with the sliced green parts of the scallions. Enjoy!