Mince the garlic and grate the ginger into a bowl. Add the sriracha, soy sauce, rice vinegar, honey, brown sugar, vegetable oil, and water. Stir to combine. Add the corn starch and stir until dissolved.
Add the chicken thighs to a large zip top bag or a shallow dish. Add half of the marinade/sauce mixture to the chicken and toss to coat (be sure to stir the sauce before adding it to the chicken. The garlic, ginger, and corn starch tend to settle to the bottom.) Refrigerate for 30 minutes. Save the unused portion of the marinade for later.
Preheat the oven to 400 degrees. Add the chicken and its marinade to a 8x8 inch casserole dish. Bake in the preheated oven for 30 minutes, basting the chicken with the juices half way through.
While the chicken is baking, add the second reserved marinade (the half that wasn't used with the chicken) to a small sauce pot. Bring it up to a simmer over medium heat while stirring. As soon as it reaches a simmer it will thicken into a nice glaze. Set the sauce aside.
When the chicken is finished baking, spread the thickened sauce over each piece of chicken. Add fresh cilantro or sliced green onions and serve.