I would suggest that to "serve 4" it might be beneficial to use more than 1 can of tuna. An ounce and a half of tuna for 4 people is pretty skimpy, IMHO. The positive nutritional value of tuna, not to mention the improved taste of the casserole, would be greatly enhanced by 1 can of tuna per person. At least then it would really be "tuna-noodle" and not "noodle-onion casserole with a hint of tuna if you really look for it". If some folks find the smell or taste of the tuna to be overpowering, the cook could always rinse the tuna before adding it to the other ingredients.

one can of tuna per person....four cans of tuna in one box of macaroni and cheese....you were kidding, right?

If you have ever used the solid pack albacore you know that it has about 3 times as much meat in it as the cheaper stuff. It really is better not only in taste but in the amount of meat to water ratio.