Preheat the oven to 190°C (170°C fan) / Gas mark 5 and grease a 23 x 33 cm (9 x 13 in) baking dish with butter or cooking oil spray.
In a large bowl, combine the cooked chicken, tinned condensed cream of chicken soup and 120 ml chicken stock. Stir until the mixture is well combined and creamy.
Spread the chicken mixture evenly in the prepared dish.
In a medium bowl, add the stuffing mix. Drizzle the melted unsalted butter over it and toss with a fork until all the crumbs are evenly coated.
Evenly sprinkle the buttered stuffing mix over the chicken layer without pressing down.
Bake for 25–30 minutes, until the topping is golden brown and crisp and the filling is bubbling around the edges.
Remove from the oven and leave to rest for 5–10 minutes before serving.