Preheat the oven to 200Â°C or gas 6. Season the chicken breasts with salt and black pepper.
Heat a large saute pan and add 2tbsp of the oil and 50g of the butter.
Fry the chicken breasts in batches for 1-2 mins on each side until golden. Put them into a deep-sided oven tray and roast for 25 mins until the chicken is cooked through.
Add the remaining butter and oil to the pan and cook the onions for 4-5 mins until softened but not coloured. Add the flour and mustard to the pan and cook for another 2 mins. Add the apples and button mushrooms and cook for 1 min. Pour in the chicken stock and bring to the boil, then pour in the cider. Bring back to the boil and cook for 5 mins. Add the cream and sage, cook for another 5 mins, then season with salt and black pepper.
Take the chicken out of the oven and pour the sauce into the dish to cover the chicken completely. Preheat the grill to high. Sprinkle the cheese over the chicken and place under the grill for 5 mins until the cheese is melted, golden and bubbling. Serve with jacket potatoes topped with a knob of butter.