In a mixing bowl, combine the soy sauce, white vinegar, half of the minced garlic, and bay leaves.
Place the chicken thighs in the marinade, cover, and refrigerate for at least 20 minutes (or overnight for deeper flavor).
Heat the vegetable oil in a large skillet over high heat. Remove the chicken from the marinade (reserve the marinade) and sear the chicken on both sides until browned, about 2–3 minutes per side, then transfer to a plate.
Reduce heat to medium, add the diced onion and the remaining minced garlic to the same skillet, and sauté until softened and fragrant, about 2–3 minutes.
Add the reserved marinade, water, brown sugar, and whole black peppercorns to the skillet and bring to a simmer, scraping up any browned bits from the pan.
Return the seared chicken to the skillet, nestling pieces into the sauce. Simmer uncovered, turning occasionally, until the chicken is cooked through and the sauce reduces to a thick syrup, about 12–15 minutes.
If the sauce needs further thickening, remove the chicken and continue reducing the sauce until it reaches the desired consistency, then return the chicken to glaze with the sauce.
Garnish with sliced green onions and serve.