In a large bowl, combine the julienned onion, smashed garlic, dark soy sauce, coconut vinegar, bay leaves, peppercorns, palm sugar, and water; whisk until the sugar dissolves and the mixture is well combined.
Divide the marinade into two equal portions and refrigerate one portion for later use.
Add the chicken pieces to the other portion of the marinade, cover or seal in a plastic zip bag, and marinate in the refrigerator for at least 12 hours and up to 24 hours.
Heat the peanut oil in a large frying pan or rondeau pot over medium heat for about 2 minutes.
Remove the chicken from its marinade (discard that marinade) and add the chicken to the pan skin-side down; brown for 10–12 minutes without moving until well browned.
Flip the chicken pieces and cook for an additional 2 minutes to brown the other side.
Pour the reserved marinade into the pan, reduce heat to low, cover, and simmer for 20–30 minutes until the chicken is cooked through and tender.
Remove the chicken to a serving platter and spoon some of the braising liquid over the top; garnish with sliced green onions if desired and serve.
