Place the potatoes in a saucepan and cover with cold water. Bring to the boil and cook for 15-20 minutes, or until just cooked. Drain well and set aside. Preheat the oven to 180ºC fan.
Meanwhile, spray a large shallow casserole dish with low-calorie cooking spray and place over a medium heat. Add the onion and fry gently for about 10 minutes, until soft and lightly golden.
Increase the heat to medium-high and add the gammon. Cook for 10 minutes, until cooked through and slightly golden.
Add the stock and milk to the pan and stir. In a small bowl, mix the cornflour and cold water to form a smooth slurry. Pour into the pan and simmer for 3-4 minutes — stirring constantly, until the sauce has thickened.
Remove from the heat and stir in the cheese, mustard, cooked potatoes, and peas. Season with freshly ground black pepper, to taste. Sprinkle the dried stuffing mix over the top and place in the oven for 15-20 minutes, until bubbling and crisp and golden on top. Serve!