Salmon fish pie

1kg potatoes,peeled and diced
30g butter
30g plain flour
400ml milk,plus 4 tbsp
3 tbsp fresh parsley,chopped
1 tbsp oil
150g mushrooms,sliced
100g peas
400g fresh salmon fillets,skinned and cubed

Cook potatoes till soft.Meanwhile,melt butter in a pan and stir in flour,gradually add 400ml milk and cook,stirring,until you have a smooth thickened sauce.Stir in parsley.Drain potatoes and mash with the remaining milk.Heat the oil in a frying pan and cook mushrooms until softened,add salmon and peas and stir.Preheat oven to 200c/400f/gas 6.Pour salmon,mushrooms,peas and sauce into a ovenproof dish top with mash and bake for 25min or until potatoes are golden.