Combine the oyster sauce, soy sauce, cooking wine, salt, sugar, white pepper and cornflour in s small bowl.
Mince the ginger and add it to the bowl. Stir to combine.
Cut the white and red onions into slices. The thickness is up to how crunchy you like your onions. Slice them more thinly if you want them to melt into the dish, or cut thicker slices if you prefer crisp bites of onion. Mince the garlic.
Debone the chicken thighs and trim any skin and excess fat. Do both for a better eating experience. To remove the bone, angle your knife towards it and slice down both sides. When the bone is more exposed, pull it up vertically and slice around it to release it from the flesh. Slice through the thicker parts of the flesh for even cooking. If you don’t, the thinner parts of the chicken will finish cooking before the thicker parts do.
Dry each piece of chicken with a clean towel or paper towel, then place them into the bowl of marinade. Coat each piece thoroughly, then set aside for about 10 minutes. For maximum penetration of the flavor into the chicken, keep the meat-side down facing the marinade (as opposed to the skin side which wouldn't absorb the marinade as well).
When they’ve finished marinating to your satisfaction, spread an even layer of cornflour on a plate, and coat each side of the thighs with a thin layer of cornflour. Gently shake off any excess cornflour.
Heat a skillet or flat-bottomed wok on high heat. Once it’s hot, add theoil.When the oil’s shimmering, turn the heat to medium-low, and place each piece of the chicken skin-side down. Cook until golden brown, or about 2-3 minutes.Flip the chicken over and cook the other side until it’s also golden brown, or about 5-6 minutes.When both sides are beautifully golden brown, flip all the pieces back to the skin side and cook for another 1-2 minutes to make it crispy. Make some room by pushing the chicken to the sides of the pan. Add the minced garlic to the now-vacated middle of the pan, and cook until fragrant, or 10 seconds.
Add the sliced onion and spread it out in the pan as you cook until that’s also fragrant, or 20 seconds.
Pour the cooking wine into the pan and immediately cover with the lid. This allows the steam to cook everything further while melding all the aromas and flavors together.Turn the heat to high to reduce the sauce down. Cook covered like this for just 30-40 seconds. Uncover the pan and sprinkle the garlic salt over the chicken for a final touch of seasoning. Give everything a quick mix to distribute. Plate your creation by creating a bed of onions on the bottom, perching the chicken thighs on top, and decorating with the most beautiful pieces of onion on top. Enjoy!