Separate celery leaves from stalks. Remove strings from the stalk and slice thinly at an angle. Chop leaves into bite-sized pieces. Slice king oyster mushrooms thinly at an angle after halving them lengthwise.

Make a slit in the mentaiko and extract the contents.

Heat mayonnaise in a frying pan over medium heat. Add celery stalks and king oyster mushrooms, stir-frying until softened.