In food processor, pulse lemongrass, scallions, garlic, and ginger. While food processor is running, slowly drizzle in oil until emulsified. Once mixture is emulsified, transfer mixture to a bowl.
Add rice wine vinegar, fish sauce, and sugar and mix. Pour half of marinade over chicken and mix well. Reserve the remaining half of marinade. Cover and marinate for at least 6 hours, but overnight is best.
When you are ready to cook chicken, remove from marinade and grill.
In a separate saucepan, put remaining half of marinade. Cook on low, until reduced by a third.
When chicken is done, spoon reduced ginger lemongrass sauce over cooked chicken.
To serve, smear one tablespoon chili-mayonnaise on bread and toast.
Arrange chicken on toasted bread, add a large pinch of cilantro, a small pinch of holy basil, 2 ounces each of pickled daikon and pickled carrots, 2 cucumber slices, and a generous pinch of crispy shallots.
*For crispy shallots, finely dice shallots and fry in peanut oil over medium high heat until crispy brown, about 5 minutes.