Bring plenty of water to a boil in a large saucepan over high heat. Add the frozen peas, return to a boil, then reduce heat to medium. Add the watercress, cover, and cook for 5 minutes until the watercress has completely collapsed.
Drain the vegetables well. Transfer to a blender, add the butter, ground black pepper, and sea salt. Blend until smooth. Taste and adjust seasoning as needed.
Transfer to a serving bowl or arrange on individual plates as a side dish.