Unrefined peanut oil adds a roasty-toasty note to the crunchy chili oil; that said, if you don't want to buy it just for this recipe, a neutral oil, like those listed in the recipe, will also work.
I like to add a bit of
umami powder (essentially, dried ground mushrooms plus some other umami-rich ingredients) to the chili oil, to mimic the flavor that MSG gives to store-bought crunchy chili oil. Alternatively, you could season it with
a bit of MSG (I swear it's not bad for you —
there's science to back this up.) Or, if you want to skip the step of making crunchy chili oil, you can use
lao gan mai (aka spicy chili crisp, aka angry lady sauce), which is available in most Asian grocery stores and
on Amazon.
Leftover crunchy chili oil keeps for at least 1 month refrigerated. It is excellent on everything from roasted vegetables to scrambled eggs to blah-tasting Asian takeout and/or leftovers; there's little that it won't improve.
If you have a mandoline — I use (and love)
this one, plus this
cut-resistant glove for safety — use that to slice the garlic, shallot, and celery; you'll get much more consistent results.
My absolute favorite way to cook the chicken for this soup is sous vide. The results are tender and it's shockingly easy to do. I use
these instructions from Serious Eats, cooking 2 boneless, skinless chicken breasts at 145°F for 1 hour and 15 minutes, then chilling them down in an ice bath before shredding. That said, if you don't have an immersion circulator (if you're in the market for one, I love
ChefSteps' Joule), leftover roast chicken is a good alternative, as is using some of the meat from a rotisserie chicken or poaching a couple breasts.
This soup is good as leftovers, provided you store the extra cilantro, celery leaves, and fried garlic separately from the soup itself, adding them just before eating. (You can also freeze leftover soup, keeping the same consideration in mind.)