This is one of those end-of-the-month budget meals. And yet, it is delicious. So, good stuff.
You cook some pasta (I tend to use the twirly ones) as you would normally.
In a seperate pot, make the sauce.
Throw some olive oil in the bottom. Add some chopped garlic and onion. Once the onion is translucent, add about a cup and a half of milk (or so - to be honest, I never ever measure it, but I think it's about that much) and a couple of tablespoons of flour. Stir so that it isn't lumpy. Allow to simmer gently until it just starts to thicken. Add some cheese. I say some, because the cheese is the most expensive part of the dish. The more you add, the more creamy and gooey it'll be. But it will also cost more. So you find your balance.
Once the pasta is cooked and the sauce has thickened some pour a layer of pasta (about half of what you cooked) into a casserole dish. Pour half of the sauce over the top. Add a layer of tuna. One tin is about right, though I have been known to splash out and use two sometimes (!!). Add another layer of the remaining pasta. Pour on the remaining sauce.
Grate some cheese on top (See above note about 'some' cheese).
Bake in an oven at 180C or so for about forty minutes to half an hour. Serve with a green salad, or cooked veggies like peas.
Really cheap to make. VERY tasty. :)