Combine all ingredients for tempura sauce in a saucepan and bring to a boil over medium heat. Remove from the heat and let cool to room temperature before serving.
Heat oil in a fryer to 340 degrees F.
In a large bowl beat egg yolk with water until thoroughly combined. Quickly stir in flour and cornstarch or potato flour, using as few strokes as possible to combine. It's o.k. if there are still a few lumps in the batter. Refrigerate the batter for 30 minutes.
*Nobu's tip: all tempura batter ingredients should be at room temperature when mixed. After the batter is mixed it should be refrigerated for 30 minutes before using.
Working in batches, quickly dip the green beans in the batter and, using tongs, transfer to the hot cooking oil. Cook until crisp and light golden brown, about 1 to 2 minutes. Using tongs or a slotted spoon, transfer to a draining rack or plates covered with paper towels to drain. Serve immediately.
Yield: 4 to 6 servings