In a small bowl or measuring cup, combine the ginger, garlic, lime zest, lime juice, rice wine vinegar, soy sauce, nam pla brown sugar, and serrano peppers, stirring until brown sugar dissolves.
Place the chicken thighs in a zip-top plastic bag and pour the marinade over them. Squeeze the air out of the bag and seal it tightly. With your hands work the chicken thighs into the marinade until well coated. Place the bag on a rimmed baking sheet and refrigerate for 8-10 hours or overnight.
Preheat the oven to 450°.
Slice the onion crosswise in about 1/4"-1/2" rings and arrange them on a rimmed baking sheet. Remove the chicken thighs from the marinade and pat them dry with a paper towel. Arrange the chicken on top of the onions. Bake for 25-30 minutes or until skin is crispy and meat is cooked through.
To serve, arrange the onions on a plate and top with the chicken breasts. Sprinkle with fresh cilantro, lime wedges and additional slices of serrano peppers, if desired.