These Asian-flavored sandwiches crusted with sesame seeds look particularly dramatic when lined up side by side. Toasting the sesame seeds deepens their flavor, so do not omit this step.
Heat a heavy skillet over medium-low heat. Add sesame seeds and shake the skillet gently, moving the seeds around, until they are aromatic, about 5 minutes. Set aside.
Place the onion, peppercorns, bay leaf, and thyme in a large pot. Add the chicken to the pot with just enough cold water to cover. Cover and bring to a simmer. Uncover and cook for 1 hour. Drain the chicken, reserving the cooking liquid, and set the chicken aside to cool. Discard the seasonings and refrigerate or freeze the stock for future use. When the chicken is cool enough to handle, remove the skin and discard. Shred the meat from the chicken and then chop it until very fine. Refrigerate, covered, until well chilled.
In a small bowl, mix together the mayonnaise and sesame oil. Add this to the chicken and combine well. Add the cilantro, chives, and scallions and combine well. Add salt and pepper to taste. Refrigerate, covered, until ready to use.
Spread each bread slice with butter, reserving some butter for garnishing. Spread an even layer of chicken salad onto half of the slices of bread. Top with the other half of the slices to make sandwiches. Trim the crusts. Cut each sandwich on the diagonal. Arrange the sandwiches long-edge down in a row. Spread the remaining butter along one side of the row. Press the sesame seeds into the buttered edge to cover. Garnish the platter with mizuna leaves and serve.
The chicken salad may be made a day in advance and kept in an airtight container in the refrigerator.