Step 1First make the marinade. In a large bowl, combine 1 tablespoon of the oil, the white pepper, onion, garlic, coriander leaves, mint leaves, ginger, fish sauce, chile, and sugar. Stir well until the sugar is dissolved.
Step 2Clean the chicken thighs and pat them dry. Put them in a large bowl or shallow dish and pour the marinade on top. Rub the marinade all over the chicken until each piece is evenly coated. Cover and refrigerate for 4 hours or overnight.
Step 3Remove the chicken from the refrigerator 30 minutes before cooking. Wipe the marinade off the chicken and reserve marinade. Find a heavy frying pan large enough to fit the chicken pieces in one layer. Pour the remaining 5 tablespoons oil into the pan and heat over high heat. When the oil is hot, add the chicken, skin side down.
Step 4Immediately reduce the heat to medium-low and cook the chicken for 4 to 5 minutes until skin is golden brown. Turn over and cook the other side for about 15 minutes or until done. Test the chicken for doneness by pricking it with a fork; when the juices run clear, remove the chicken from the pan and keep warm.
Step 5Pour out the fat and return the pan to medium heat. Spoon in the reserved marinade and stir to bring up the brown bits on bottom of pan cooking 2 to 3 minutes. Add one-half cup water and bring to a boil. Turn down and simmer for a few more minutes. Return the chicken to the pan to reheat if necessary.
Step 6Serve the chicken with steamed rice and garnish with fresh cilantro and mint leaves. Drizzle the pan sauce over the chicken and serve.