If cooking a fresh batch of rice, start that right away. Otherwise just warm leftover rice.
For the beef bulgogi: In a large bowl, stir together the soy sauce, rice vinegar, brown sugar, garlic, and ginger. Add the sliced beef and fold to combine. Set aside at room temperature to marinate, about 20 minutes.
For the tahini-soy sauce: Now whisk together all of the tahini-soy sauce ingredients in a small bowl. Set aside.
To cook and assemble: Once the beef is done marinating and the tahini-soy sauce is made, heat a large skillet over medium heat and add the sesame oil.
When the oil is just barely warm (not hot), add the bulgogi (both the beef and marinade). Cook for 5-7 minutes or until the liquid has evaporated and the beef is just beginning to crisp. Remove skillet from heat.
Heat a separate small skillet over medium heat. Add the toasted sesame oil.
When the oil is hot, place the bok choy cut-side down. Sear for 2-3 minutes or until the bok choy is a bit browned yet still crisp. Flip the bok choy and cook for an additional 2-3 minutes.
Divide the rice, beef bulgogi, and bok choy between four bowls. Top with tahini-soy sauce (or offer alongside). Garnish with green onions and sesame seeds if desired.
