In a bowl, mix together white pepper, salt, cornflour and oil to make the marinade. Pour over the chicken, turn to coat, let sit for 20 minutes.
Meanwhile boil the potatoes for 30 minutes. Drain well. Add 50g butter and 75ml and mash well.
In a small bowl, mix together the soup, milk, water and butter. Set aside.
Pre-heat the oven to 250 C / Gas 9.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the onion with salt for 2-3 minutes. Add the chicken and stir-fry for 3-4 minutes. Stir in the sauce and heat through, 1-2 minutes.
Whilst stirring, slowly add the cornflour slurry until the sauce is thickened. Pour into a baking dish and cover with the mashed potatoes.
Bake in the pre-heated oven for 15 minutes or until golden on top. Serve hot.