Combine the lemongrass, garlic, brown sugar, red chile, coriander, and salt in a mortar and, using a pestle, pound to form a paste. Stir in the fish sauce, soy sauce, and oil.  Transfer the marinade to a medium non-reactive bowl.

Using a sharp boning knife, trim any excess fat or remaining cartilage from the chicken thighs and discard. Dip the chicken pieces in the lemongrass mixture, turning to coat on both sides, and then nestle the pieces down into any remaining marinade. Cover and refrigerate for at least 4 hours and up to overnight. When ready to cook the chicken, remove from the refrigerator and allow to sit at room temperature for up to 30 minutes.

Position an oven rack as close to the broiler unit as possible and preheat the broiler. 

Transfer the chicken pieces to a foil-lined baking sheet and season lightly with kosher salt.  Broil the chicken until just cooked through, 4 to 6 minutes.  Remove from the oven, set aside to cool briefly, and then slice the chicken into thin strips.

Halve the bread lengthwise and spread generously with mayonnaise.  Spread a thin layer of Chicken Liver Mousse on the bottom portion of bread if desired. Drizzle the Sriracha sauce on the upper pieces of bread, to taste.  Divide the chicken pieces evenly among the bottom portions of bread, then top with the sliced cabbage, pickled carrots, cucumber slices, and jalapenos, to taste. Tear any large basil leaves into pieces and sprinkle over the sandwich, along with mint and cilantro. Top with the remaining bread, cut the sandwiches in half, and serve.