In a small bowl, combine all the ingredients for the dipping sauce (Yangnyeom Jang). Mix until sugar is dissolved. Set aside.
Generously salt and pepper both sides of the tuna steak. Add a thick layer of sesame seeds to each side, coating well.
Heat a cast iron skillet over medium high heat. When the pan slightly smokes, add 1-2 tsp of oil and swirl around in the pan.
Add the tuna steaks to the hot pan, being careful of oil splatter. Cook until the bottom is golden and crusty, about 1-2 minutes. Flip and cook on the other side until golden and crusty, another 1-2 minutes.
Transfer to a cutting board. Slice thinly, against the grain.
Add rice to two bowls. Add sliced tuna and avocado slices. Drizzle Korean dipping sauce on top of the tuna.