In a large pot, boil your pasta following the packet's instructions.

When pasta is cooked, drain and add to your tuna in the casserole dish and mix together.

Mix 100g of the grated cheese in with the pasta and tuna.

Using the same pot you boiled your pasta in, return to the stove and add a big knob of butter. Reduce heat and once butter has melted add flour to make a easy roux. Slowly add your milk a little at a time stirring constantly to remove floury lumps. When all milk has been incorporated and thickens stir in your mustard.