Note - I've been making this regularly since 1999.
Tuna now comes in 5 oz. cans, not 6 (or even the 7 I remember. Why couldn't they just leave the same size and raise the price?!) The original recipe was for white potatoes, but I have used reds and small russets too. When I say no balsamic vinegar or scallions I mean it! I've tried them at one time and they were not right for the recipe.
* did you know that the flatter the onion the milder it will be? So if you have only a pointy onion be sure, after you have chopped it, to rinse the pieces under water in a sieve.