Salmon casserole

1. White sauce:
Melt butter. When bubbling, take off heat and stir in flour, salt, pepper, and mustard. Cook for a couple of minutes. When gritty, take off heat and add milk gradually, whisking; stir out lumps and simmer until cooked.

2. Add all ingredients to ceramic dish, stir in white sauce. Grate parmesan and put dob of butter on top of the casserole and cook in the oven. Nana used to add fresh breadcrumbs. Cook in oven, until top crisps up; though Mum also does on stove top.