Preheat the grill to moderate. Cut four 1 cm slices of bread on the diagonal and toast on one side until golden, about 2 minutes. Tear the remaining bread into 1 cm pieces (you need 4 cups).
To make the stuffing, lightly coat an ovenproof frying pan with cooking spray and set over a medium–high heat. Sauté the onion, apple and celery for about 5 minutes or until soft. Add the chicken stock, thyme, dried mixed herbs and bread pieces. Cook, stirring, until heated through, about 4 minutes. Transfer the pan to under the grill and grill for about 3 minutes or until the top is browned. Stir the browned parts under and grill for 3 minutes more.
Meanwhile, to make the dressing, combine the cranberries, chicken stock, vinegar, honey, oil and orange zest in a small saucepan over a medium–high heat. Bring to a simmer and cook, stirring, for 1 minute, then remove from the heat.
For each sandwich, place a piece of toast on a plate, toasted-side down. Spoon some of the stuffing on top and cover with chicken slices. Drizzle with the warm dressing.