Heat the oil in a medium Chinese clay pot or in a casserole. Add the chiles, garlic and half of the ginger and stir-fry over moderate heat until fragrant, about 20 seconds. Add the chicken, fish sauce, salt and the remaining 2 teaspoons of sugar and stir until the chicken turns white, 3 to 4 minutes. Add the onion wedges, the remaining 1/4 cup of hot water and the caramel sauce and bring to a boil. Cover, reduce the heat to low and simmer until the chicken is cooked through and the sauce is slightly thickened, about 8 minutes. Add the scallions and cook for 3 minutes longer. Stir in the remaining ginger and remove the pot from the heat. Garnish with the cilantro and serve.