Place the chicken in a bowl. Whisk the brine to redistribute the garlic and chiles evenly in the brine. Ladle the brine over the chicken and toss. Cover the bowl with plastic film and marinate for at least 8 hours and up to 2 days before use.
For the chicken sauce: Combine 1 cup water, sugar, vinegar and fish sauce in a bowl. Whisk until the sugar is completely dissolved. Add the garlic and Thai chiles to the mixture and whisk until well dispersed. Add the Thai chile sauce and whisk to combine.
Preheat oil in a deep pot or in a deep-fryer to 375 degrees F.
Remove the chicken from the brine and transfer to a bowl (be sure to wear gloves while handling raw chicken). Add the cornstarch and toss the chicken until well coated. Place the chicken in a sieve set in an empty metal bowl and shake the chicken until excess cornstarch falls into the bowl underneath.
Place the chicken into a fryer basket and drop into the oil. Fry the chicken until crispy and golden brown. Make sure to shake the basket so that the pieces cook evenly. Separate using tongs if needed to prevent sticking.
While the chicken is frying, combine the jalapenos and onions in a metal bowl.
When the chicken is done frying, drain excess oil on paper towels and let rest. Transfer to the bowl with the vegetables and season with the salt. Pour 1/4 cup of the chicken sauce over and toss until evenly coated. The chicken and vegetables should have a nice sheen, but the chicken should still be very crisp.