Heat oven to 350 degrees F. Spread sesame seeds on a baking pan. Bake until fragrant and golden, about 5 minutes. Transfer seeds to a bowl to cool.
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. Then 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 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 to form a horizontal pyramid. Spread the remaining butter along 1 side of the pyramid. Spread the toasted sesame seeds along the buttered edge to cover. Garnish the platter with mizuna leaves and serve.
Combine the oils in a large glass measuring cup. In the bowl of a food processor, add the eggs, mustard, and 3/4 teaspoon of salt. Process until the mixture is foamy and pale, about 1 1/2 minutes. With the machine running, add the oil, drop by drop, until the mixture starts to thicken (about 1/2 cup of oil total)/ do not stop the machine at this point or the mayonnaise may not come together.
Add the remaining oil in a slow, steady stream. When all of the oil has been incorporated, slowly add the lemon juice. Taste and adjust the mayonnaise for seasoning. Fresh mayonnaise can be kept, refrigerated, in an airtight container for up to 5 days.
*RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.
Yield: 2 1/2 cups
Use light olive oil to make the mayonnaise for delicately flavored foods such as tea sandwiches Use richly flavored extra-virgin olive oil for a more assertive version. .
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