Put the chicken and onions in a saucepan with salt, garlic and peppercorns. Add just enough water to cover. Bring to a boil, reduce to a simmer and cook until the chicken is cooked through, about 8 minutes.
While the chicken is cooking, prepare the dressing by whisking together lime juice, fish sauce, rice vinegar and sugar. Add chillies and stir to combine.
Remove the chicken immediately from the pot. Reserve cooking liquid.
Shred the chicken with your fingers, as soon as it is cool enough to handle.
Season the chicken with salt and liberally with black pepper.
Add 1/2 cup of the cooking liquid to the chicken. Strain the cooking liquid and store in the refrigerator for future use, whenever chicken stock is needed.
Put the cabbage and herbs in a large bowl with the shredded chicken. Add dressing and toss to combine.
Let stand for at least 15 minutes before serving.
Serve chilled or at room temperature.