Cut each chicken thigh into three equal pieces. Place in a large bowl with the ginger, two-thirds of the oil, and salt and pepper. Cover and marinate for about 6 hours in the refrigerator.
Heat a large, heavy-based frying pan and pour in the remaining oil. Fry until the meat is colored, about 2 minutes, then drain away the excess oil.
Add the fish sauce, cover the pan with a lid, and steam until the sauce has dissolved, about 1 minute.
Now add the prepared caramel to the pan and cook until the caramel has thickened and coated the chicken in a rich golden color, about 2 minutes, stirring occasionally.
To serve, arrange the chicken pieces on a serving platter and garnish with watercress.
To make caramel, place 2/3 cup sugar in a large, heavy-bottomed saucepan, add 4 tablespoons water, and bring to a boil over medium heat. Leave to caramelize, about 10-15 minutes. The caramel will reach a temperature of almost 180 degrees F, so care and attention are required. To caramelize sugar correctly, invest in a proper sugar thermometer. Do not leave the pan unattended when sugar is caramelizing.
Recipe adapted from Lemongrass and Lime (Ten Speed Books, 2001) by Mark Read and Christian Benians