First, combine the vinegar, soy sauce, bay leaves, ginger, garlic and black pepper. Add the chicken thighs (or chicken pieces) into this marinade, cover and refrigerate for at least 2 hours, but preferably overnight. This helps infuse the flavours into the chicken and gives it a beautifully rich dark colour.
Before cooking, remove the chicken from the fridge half an hour earlier so that it comes to room temperature.
Heat the oil in a wide-bottomed pan. Remove the chicken pieces from the marinade and shake off any excess. Reserve the marinade to be used later. Add the chicken pieces into the pan, making sure not to over-crowd the pan. Allow the chicken to sear and colour on both sides for a few minutes, then remove them to a plate and set aside.
In the same pan, add the onion slices and stir them until lightly golden. Then add the red chillies if you are using them, followed by the reserved marinade and the water. Bring to a gentle simmer, and then add in the chicken pieces and stir them in. Turn the heat down to medium low, cover the pan and allow the chicken to cook for about 25-30 minutes, giving everything an occasional stir. Make sure to turn the pieces a couple of times in the sauce to ensure they acquire that lovely rich colour on all sides.
If you like, you can add some boiled eggs into the sauce.
Once the chicken is tender and cooked through, turn the heat off and let the dish sit for at least 10 minutes before serving, to allow the flavours to settle and mellow.
Serve over white rice.