Pierce the potatoes and cook for 5 minutes each in the microwave, until soft.
Scoop out the centres of the potatoes into a bowl and season with salt and pepper to taste. Mash until smooth. Reserve the potato shells for later.
Spray a frying pan with low-calorie cooking spray and set on a medium heat. Add the onions and fry for 5 minutes. Then add the mushrooms and continue to fry for a further 5 minutes.
Add the chicken and continue to cook, until browned on all sides.
Add the stock cube to the water and stir until dissolved. Pour into the pan, along with the white wine vinegar.
Combine the cornflour and cold water, and mix until smooth. Pour into the pan and lower the heat. Simmer for 10-15 minutes, until the chicken is cooked through and the sauce has reduced.
Add the quark (or reduced-fat cheese) and parsley, and stir until dissolved.
Preheat the oven to 180ºC.
Spoon the chicken filling into the potato shells and place onto a baking tray.
Top each one with the mashed potato, then cook in the oven until they are golden brown and the filling is piping hot.