Heat the vegetable oil in a 12-inch skillet over medium-high heat. Nestle the bok choy halves in the skillet, cut sides down (it’s okay if they come up the sides of the skillet—they will cook down). Season with salt and pepper. Cook until browned on the bottom, rearranging them as needed, about 2-4 minutes.
Flip the bok choy over, season with more salt and pepper, and cook 2-3 minutes on the other side. Add the water and hoisin sauce. Cook, stirring and flipping the bok choy occasionally, until it’s tender and coated in the glaze, about 3-4 minutes longer (if the bok choy halves are on the larger side you might need to add another splash or two of water and cook a bit longer).
Transfer the bok choy to a platter and drizzle any glaze from the pan over top. Sprinkle with sesame seeds and gochugaru, if you'd like.