Pat the yellowtail dry, season with salt and pepper, and cut into bite-sized pieces.

Slice the leek diagonally into 2cm pieces. Cut the king oyster mushrooms into thirds lengthwise, then slice vertically into quarters.

Heat half of the salad oil (1/2 tbsp) in a frying pan over medium-low heat. Add the yellowtail and cook until both sides are browned, flipping occasionally. Remove and wipe off excess oil.

Add the remaining salad oil (1/2 tbsp) to the pan and heat over medium-low. Add the leek and cook, rolling it until browned. Add the mushrooms and stir-fry until softened.

Return the yellowtail to the pan, add the soy sauce, sake, and butter, and stir-fry to combine.