Cook the past according to the packet instructions.
To make the sauce, melt the butter in a large pan, add the flour and whisk to get a thick paste.
Pour in the milk whisking continuously until it thickens.
Add half of the grated cheese, dijon mustard and black pepper, and mix well.
When the pasta is cooked, drain in well, then add it to the sauce and give it a good mix.
Transfer everything to an ovenproof dish, add the cooked peas and well-drained tuna, and mix.
Scatter over the remaining cheese, and bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 20 minutes until golden and bubbling hot.
Garnish with fresh parsley.