Fill a large pot with water and place over high heat. Fit a steaming basket over the top of the pot. Add the bok choy to the steaming basket and cover. Bring the water to a boil.
Once the water is boiling, briefly remove the steaming basket and add the noodles. Boil the noodles for 2-3 minutes or until tender.
Remove the steaming basket from the pot and set aside. Do not remove the cover.
Drain the noodles, reserving 1/4 cup of the cooking liquid, and rinse them under cold water. Leave them to drain further while you make the sauce.
In a medium-sized bowl whisk the soy sauce, sesame oil, vinegar, Sichuan peppercorns, honey, tahini, and 1/4 cup of the reserved chili oil together until smooth and creamy. Stream in the reserved cooking liquid while whisking constantly.
Transfer the noodles to a large bowl and pour the sauce over top. Toss to coat.