Combine the chicken, soy sauce, vinegar, garlic, bay leaves, and peppercorns in a bowl. Marinate for 15 minutes while you prepare the pan.
Heat the oil in a large Dutch oven or skillet over medium-high heat. Lift the chicken from the marinade, reserving the liquid, and brown the pieces on both sides.
Braise the chicken:
Pour the reserved marinade into the pan and let it simmer for 2 minutes without stirring.
Add the water and brown sugar if using. Bring to a gentle simmer, cover, and cook for 30 minutes, turning the chicken once.
Uncover and simmer for 10 to 15 minutes more, until the chicken is tender and the sauce is glossy and slightly reduced.
Serve:
Taste the sauce and add a splash of water if it is too concentrated, or simmer a little longer if it is too thin.
Discard the bay leaves. Spoon the chicken and sauce over steamed rice and finish with scallions.
Notes
Do not skip the short vinegar simmer before adding the water; it helps the sauce taste balanced rather than sharply acidic.
For crispier chicken skin, transfer the cooked thighs to a sheet pan and broil briefly before serving.
Leftovers reheat gently in their sauce and keep refrigerated for up to 4 days.