Melt half the butter in a frying pan and lightly brown the chicken pieces.
Transfer to a caserole dish and roast for approximately 1 hour until cooked through.
Prepare the sauce by melting the remaining 1 oz butter in a saucepan and gently cooking the onion until soft and transparent. Stir in the flour and cook gently for 2 to 3 minutes, stirring all the time.
Remove the pan from the heat and stir in the milk and cider gradually.
Return to the heat and bring to the boil, stirring all the time until the sauce thickens.
Cook for 2 to 3 minutes. Remove from the heat and stir in three quarters of the cheese, the mustard and salt and pepper to taste. Pour the sauce over the cooked chicken, sprinkle with the remaining cheese and brown in a hot oven or under a grill.
Serve hot with green vegetagles and jacket potatoes.