In a large bowl, whisk together rice vinegar, soy sauce, and peppercorns. Add chicken thighs, coating them well. Cover and marinate for at least 20 minutes, up to overnight.
Heat 1 tablespoon of oil in a wide skillet over high heat. Remove chicken from marinade, letting excess drip back (reserve marinade). Cook chicken smooth side down for 2-3 minutes until golden brown. Flip and cook for another 2 minutes. Remove chicken to a plate.
Lower heat to medium-low, add remaining oil, then sauté onion and garlic for about 2 minutes until fragrant and onion softens.
Pour in reserved marinade, add bay leaves and sugar. Bring to a boil, scraping up browned bits from the pan bottom. Reduce heat and simmer for 3-4 minutes.
Nestle browned chicken thighs smooth-side down into the sauce. Simmer uncovered for 10 minutes, then flip and spoon sauce over them. Continue simmering for another 10-15 minutes until sauce thickens and chicken is cooked through.
If sauce isn’t thick enough, remove chicken, reduce sauce to a glaze consistency, then return chicken to warm through.
Serve chicken with sauce over steamed white rice, garnished with green onions.
Notes
Store leftovers in an airtight container in the fridge for up to 4 days, or freeze for up to 3 months.