Cut the chingensai into 3cm pieces, separating stems and leaves. Quarter the stem pieces lengthwise. Trim the shimeji mushrooms and separate them. Remove the pit from the umeboshi and finely chop it.

Layer the chingensai stems, shimeji mushrooms, and chingensai leaves in a microwave-safe container. Cover loosely with plastic wrap and microwave at 600W for 3 minutes. Drain excess water and let cool slightly.