Cut the celery diagonally into thin slices and remove the strings. Pit and finely chop the plum.

Make shallow cuts in the chicken tenderloin in three places.

Place the chicken in a microwave-safe container, sprinkle with sake, cover loosely with plastic wrap, and microwave at 600W for 1 minute 30 seconds. Leave covered for 5 minutes to cook through with residual heat, then cool and drain excess moisture.