Step 1Bring a large pot of water to a boil. Add 3 tablespoons salt, return to a boil, then add the French beans. Boil until just tender, about 5 minutes. Drain well, rinse under cold water until cool, then drain again.
Step 2While the beans cook, whisk the miso, vinegar, oil, honey and a pinch of salt in a large bowl until smooth. Add the beans and half the jalapeño and fold until evenly coated.
Step 3Transfer to a serving platter and garnish with the remaining jalapeño and sesame seeds.