1. Heat the olive oil in a large skillet over medium heat, add the ground mustard seeds and cook, stirring constantly, until the seeds pop, 1 to 2 minutes. Add the watercress, garlic, tarragon, marjoram, nutmeg, and pepper and cook, stirring, for 5 minutes.
2. Carefully transfer the cooked watercress mixture to a blender, add the remaining ingredients, and process until smooth, holding down the blender cover with a towel at first to prevent eruption. Reheat gently if necessary.