Fill a large pot with salted water for the pasta and bring to the boil. In a medium sized saucepan place 40gm of butter and melt over medium heat.
Add the dried pasta to the water and cook according to the manufactures instructions.
Once the butter has melted add the flour to the butter and return to the stove and cook the flour out (approx 1 minute) stirring continually.
Remove the saucepan from the heat and add approx 1/4 of the milk, return to the heat and cook until thick and smooth, add the remaining milk and continue to cook until the sauce is smooth, thick and creamy. Set aside.
In a large mixing bowl place the drained tuna, peas, capers and seasoning, mix together until well combined and set aside. Add Dijon mustard a pinch of cheese and remaining butter to the sauce and pour the sauce over the tuna mix.
Drain the pasta well and add it to the tuna and sauce mix, stir all the ingredients together until the sauce is costing the pasta and the tuna and peas are thoroughly incorporated. Taste the mix and add seasoning if required.
Transfer the pasta to a gratin dish and top with grated cheese, place the dish into a pre-heated oven combi setting 185 °C for 18-20 minutes until bubbling and golden on top.
Serve hot from the oven with a crisp green salad.