In a large saucepan slowly bring the milk to near boiling. Mix the corn flour with some additional milk until fully combined. Stir into the milk while stirring and becomes thick and starts to boil, turn down.
Add the onion and stir through. Add the cheese and stir until fully melted in. Gently stir in the tinned salmon.
Transfer to a baking dish then sprinkle with the breadcrumbs and grated cheese. Bake in a moderate oven until the cheesy crust has melted, about 15 minutes.