Combine the chicken thighs, soy sauce, vinegar, crushed garlic, bay leaves, whole black peppercorns, and brown sugar in a large bowl. Cover and refrigerate for at least 30 minutes, or overnight for best results.
Remove the chicken from the marinade and pat dry with paper towels. Reserve the marinade.
Heat the cooking oil in a large skillet or Dutch oven over medium-high heat. Sear the chicken on both sides until golden brown, about 2-3 minutes per side. Work in batches if needed.
Pour in the reserved marinade and water. Bring to a boil, then reduce the heat to low. Cover and simmer for 30 minutes, flipping the chicken halfway through.
Remove the lid and let the sauce simmer uncovered for 15-20 more minutes until the sauce has reduced to a thick, glossy consistency.
Serve the chicken over steamed white rice and spoon the reduced sauce over the top. Garnish with sliced green onions.