In a large saucepan, combine 4 cups of water, sugar, and salt. Bring to a boil, stirring until sugar and salt are dissolved.
Remove from heat and refrigerate until completely cold. Add onion, garlic, bay leaves, and peppercorns.
Add chicken to pot to fully submerge in solution. Refrigerate overnight.
Drain chicken from the brine. Rinse well and pat dry.
Steam for about 30 to 40 minutes or until cooked through but not falling apart.
Remove chicken from heat and refrigerate, uncovered, to cool completely. Brush all over with fish sauce.
In a deep, heavy-bottomed pot, heat about 6-inches deep of oil (enough to fully cover chicken during frying) to 350 F.
Gently add chicken and deep fry, turning as needed, for about 5 to 7 minutes until lightly browned. Remove from pot and drain on paper towels. Allow to rest for about 10 minutes.
Reheat cooking oil back to 350 F. Gently add chicken back into hot oil and deep fry for about 5 to 7 minutes or until golden brown and crisp.
Remove chicken from pot and drain on a wire rack. Allow to rest for about 5 minutes before cutting into serving portions.