Preheat the oven to 350 degrees F. Grease a rectangular baking dish and keep it aside.
Cook the pasta according to the package instruction minus 1 minute and keep it aside.
In a large dutch oven, add butter; once the butter is melted, sprinkle the flour and garlic. Cook for a minute.
Slowly whisk in the milk and cook until the sauce is smooth without any lumps and starts to thicken.
Season the sauce with ground mustard, salt, and pepper.
Add the cheeses; cheddar cheese (save ¼ cup for later use) and parmesan cheese and continue to cook until the cheese is melted.
To the creamy sauce, add the cooked pasta, canned tuna, peas, and frozen corn. Mix until the sauce is evenly coated.
Transfer them to the prepared baking dish and top the pasta with the remaining ¼ cup of cheese.
Bake them for 15-20 minutes until the cheeses are melted.
Once done, garnish the dish with fresh parsley.
Transfer the pasta to a bowl and serve while it is still warm.