Microwave the potatoes for 10 mins on High, turning after 5 mins, and heat oven to 220C/fan 200C/gas 7. Meanwhile, heat the oil in a frying pan, then fry the mushrooms over a high heat until golden. Stir in the cornflour, gradually add 100ml milk, then simmer to a smooth sauce. Season to taste, then stir in the chicken and parsley.
Scoop most of the potato from the skins, then mash with the remaining milk and some seasoning. Spoon the chicken filling into the shells, top with the mash, then bake for 10 mins until golden and the skins have crisped a little. Serve with green veg or a salad.