Place sesame seeds in a shallow plate and place the cod in the seeds to coat on both sides.
Set a nonstick skillet on medium-high heat, and once hot add avocado oil. Place the cod in the skillet and cook on each side for about 1 minute but no more than 2 minutes (since cod cooks very fast).
Remove the cod from the skillet, then add the halved bok choy with the sliced part facing down into the skillet. Squeeze in a little lime to create steam, then cover and cook for 3 to 5 minutes, until the bok choy is seared and slightly wilted. You want a crisp tender result – tender to bite with a little crunch. Set aside.
Mix together the ingredients for the sauce, then drizzle over the cod and the bok choy.