Combine chicken and marinade ingredients in a bowl. Marinate for at least 20 minutes, or up to overnight.
Heat 1 tbsp oil in a skillet over high heat. Remove chicken from marinade (reserve marinade) and place in the pan. Sear both sides until browned – about 1 minute on each side. Do not cook the chicken all the way through.
Remove chicken from skillet and set aside.
Heat the remaining oil in a skillet. Add garlic and onion, cook for 1 1/2 minutes.
Add the reserved marinade, water, sugar, and black pepper, bring it to a simmer, then turn the heat down to medium-high.
Simmer for 5 minutes.
Add chicken smooth side down. Simmer uncovered for 20 to 25 minutes (no need to stir), turning chicken at around 15 minutes, until the sauce reduces to a thick jam-like syrup.
If the sauce isn’t thick enough, remove the chicken onto a plate and let the sauce simmer by itself - it will thicken much quicker - then return the chicken to the skillet to coat in the glaze.