Mix the cabbage with a splash of the leftover giardiniera pickling liquid and massage in using your hands, along with a pinch of salt and pepper. Set aside. Roughly chop the remaining giardiniera and stir through the mayonnaise, adding another splash of the leftover pickling liquid. Season to taste.

Spread some of the mayonnaise over the slices of the bread. Fill your sandwiches with the remaining chicken breast, cut into slices, with more mayo on top, then a handful of the cabbage.