Bring a medium size pot of water (about 6 cups), salted with 1 tablespoon kosher salt, to boil.
Put the peanut butter, honey, rice vinegar, soy sauce, and mirin in a medium bowl. Whisk until smooth and set aside.
Rinse the watercress, drain and separate the leaves from the stems.
Roughly chop the stems and add to the boiling water along with the leaves. Cook until the stems are tender but yielding a soft crunch (about 2-3 minutes).
Drain, rinse under cold water and softly squeeze out excess water. Gently pat the watercress dry with a paper towel and add to a mixing bowl. Pour the dressing over the watercress and toss until the watercress is evenly coated.