Grind 2 tablespoons of the sesame seeds in a spice grinder or with a mortar and pestle until they reach a powdery consistency. Combine ground sesame seeds, vinegar and miso in a small bowl and whisk until well blended.
Bring a large saucepan threequarters filled with water to a boil. Add green beans and salt; boil until green beans are tender to the bite, 6 to 8 minutes. Drain well. Place green beans in a shallow serving bowl and spoon sauce on top. Garnish with remaining 1 teaspoon sesame seeds.
Whole World Vegetarian