Preheat oven to 200°C fan. Boil pasta 1 minute less than packet directs. Drain.
Melt butter in a saucepan over medium. Whisk in flour. Cook 1 minute.
Slowly whisk in milk. Cook 5 minutes until thick. Off heat, stir in 100g tasty cheese, parmesan, mustard and nutmeg.
Combine pasta, sauce, drained tuna, frozen peas in a baking dish. Salt and pepper to taste.
Top with remaining 50g cheese and breadcrumbs. Bake 20 minutes until golden and bubbling.
Top with parsley before serving.