Preheat your oven to 180°c/160 fan/350f and put a large pot of salted water on to boil.
Add the pasta to the pot and cook for 2 minutes less than recommenced on the packet instructions. Add the leeks to the pot for the final 2 minutes of cooking.*
Scoop out ¼ cup (4 tablespoons) of the cooking water from the pasta, then drain it.
In the same (warm) pot you used to cook the pasta, add the cream, the reserved pasta water, two thirds of the cheese (100g) plus the pepper, garlic powder and salt.
Mix well, then add in the tuna, pasta and parsley and stir again.
Transfer everything into a baking dish, give it another stir to make sure all the ingredients are evenly distributed, then sprinkle with the remaining cheese.
Bake in the oven for 15-20 minutes, until the cheese on top is golden brown.
Sprinkle with some extra chopped parsley and serve.
Notes
*for example, if the pasta packet recommends cooking for 10-12 minutes, you should cook the pasta for 6 minutes then add the leeks and cook for a further 2 minutes, 8 minutes in total.