Peel the potatoes and slice them to the thickness of a £1 coin. Add to a pan and cover with cold water. Bring to the boil, lower the heat, and simmer for 10 minutes. The potatoes should be just soft and holding their shape. Drain well and leave to one side.
Spray a large pan with low-calorie cooking spray and set on a medium heat. Add the sausages and brown on all sides. Remove to a plate.
Respray the pan and lower the heat, then add the onions. Cook for 15 minutes, stirring occasionally. The onions should be caramelised, softened, and brown.
Add the mushrooms and fry for 5 minutes.
In a jug, combine the apple juice, water, stock pot, and cornflour mix — until smooth. Core the apple and dice, no need to peel.
Preheat the oven to 190ºC. Then add the thyme and apple to the pan. Add the sausages back to the pan, and pour over the apple and stock mixture. Simmer for 10 minutes, until reduced and glossy.
Scrape into an ovenproof dish and top with the sliced potato, overlapping each slice with the previous. Then spray with low-calorie cooking spray and sprinkle over some pepper.
Pop into the oven for 20-25 minutes until the potatoes are browned and cooked through. Serve.