Melt butter in saucepan, fry onion.
Add flour. Cook for 1 minute. Add bayleaf, gradually add milk while stirring constantly until sauce thickens. Add cheese, reserving 2 (or 3) tablespoons for topping.
Add salmon and lemon juice, remove the bay leaf, and stir well. You can add water if it’s too thick, but it’s usually fine.
Place mixture in a casserole dish, sprinkle equal amounts of breadcrumbs and grated cheese on top, dot with the extra butter.
Place in oven at 180 degrees for 20 - 30 minutes until browned on top.
Serve with mash potato