Heat butter in a 3-quart cast iron pot over medium heat. When butter foams, add ginger and cook until fragrant, about 1 minute. Add sake and bring to a boil. Add mussels, stir to coat, cover, and steam until mussels open, about 3 to 5 minutes.
Using a slotted spoon, remove mussels to a serving plate. Add miso to sauce and stir until smooth. Remove from heat, stir in watercress, pour over mussels, and serve.
Serves: 2; Calories: 480; Total Fat: 19 grams; Saturated Fat: 9 grams; Protein: 47 grams; Total carbohydrates: 26 grams; Sugar: 5 grams; Fiber: 5 grams; Cholesterol: 126 milligrams; Sodium: 1809 milligrams