Trim watercress from the root end, leaving stems and leaves intact. Run your hands through the stems to see if any are overly tough and woody. If you find any that fit that description, trim and discard. Be sure to give your watercress a good rinse to remove dirt and grit. Dry thoroughly.

Trim the root end and tips of the green onion. Cut in quarters.

Add watercress, green onion, serrano chile, lime juice, a pinch of kosher salt and a pinch of white pepper to a food processor. Pulse a few times. Add mayonnaise and process until smooth. Season to taste with additional kosher salt and white pepper if needed.