Pat the yellowtail fillets dry and cut them into thirds. Season with salt and pepper.

Cut the cabbage into bite-sized pieces and trim the base of the shimeji mushrooms, separating them by hand.

Mix sugar, soy sauce, and gochujang in a bowl to create the sauce.

Heat half of the sesame oil in a pan over medium-low heat. Fry the yellowtail fillets until golden on both sides, then remove and wipe the pan clean with a paper towel.

Add the remaining sesame oil to the pan and stir-fry the shimeji mushrooms until softened. Add the cabbage and stir until evenly coated with oil.