Melt 20g butter in a frying pan and add the mushrooms. Sauté over a high heat until starting to brown and any liquid has evaporated. Put to one side.
Melt the remaining butter in a pan and stir in the flour.
Take the pan off the heat and gradually stir in the milk and cider to get a smooth sauce. Return to the heat and, stirring all the time, bring to the boil.
Simmer gently for 2 minutes and season with salt and white pepper. Add the parsley.
Stir in half of the cheese and allow to melt.
Add the mushrooms, stir well and tip into a baking dish.
Heat oven to 200C/fan 180C/gas 6.
Scatter over the remaining cheese, walnuts, and breadcrumbs.
Place in the oven and cook for 10 mins until the cheese is melted and golden.