Scoop out the cooked chicken and transfer to a bowl (discard the onion, garlic and peppercorns).
Bring the broth to a boil then drop in the cabbage wedges and sweet corn.
When the broth is boiling once more, lower the heat, cover the pot and simmer the cabbage and corn for ten minutes.
Taste the broth and add more fish sauce (the cabbage and corn would have absorbed the saltiness of the broth).
Drop in the potato cubes, cover and simmer for another ten minutes.
Add the squash and sweet potatoes, and simmer for another five to seven minutes.
Taste the broth once more and add more fish sauce, as needed.
Return the chicken to the pot and simmer with the vegetables for a minute or so.
Give the broth a final taste and, if needed, stir in more fish sauce.