Heat the oven to 180°C/160°C (fan) or gas mark 4.
Place a large pan of water over high heat and bring to a boil. Lightly season with sea salt, then add the pasta and cook for about 7-9 minutes.
Look at the instructions on your pasta packaging. See how long it tells you to boil the rigatoni for, and drain the pasta three minutes before this time.
For the white sauce, melt the butter in your saucepan and stir in the plain flour to make a “roux”, which is basically the base of the sauce.
Once you have cooked the roux for a minute (don’t let it brown), slowly stir in 600ml of milk, and stir, gently but continuously, until the sauce thickens, about 5-7 minutes.
If the sauce starts bubbling, turn the heat to low and continue to stir until the sauce has reached a silky creamy consistency.
Turn the heat off and season with sea salt, freshly cracked black pepper and freshly grated nutmeg to taste.
Once the white sauce is ready, stir in 2/3 of the cheddar and 2/3 of the parmesan cheese, and set the sauce aside.
By this time, your rigatoni should be ready and super al dente.
Drain and transfer the pasta into a large bowl, add in the tuna, sweetcorn and peas.
Stir in ½ of the white sauce and mix until combined. You can also add some chopped parsley if you like.
Transfer the ingredients in a 6x8 inches baking dish, and top with the remaining white sauce.
Sprinkle the remaining cheddar and parmesan cheese on top, and your pasta is ready to hit the oven.
Bake for around 15-20 minutes, or until the cheese on top is golden brown. If you want some nice crunchy brown spots, set the baking dish under the grill for 2-3 minutes extra.