Although mayonnaise suits tuna well, it does not make a salad... I'm a minimalist when it comes to food, but I still like tuna salad. I usually mix:

Dice the onion and mix it all in a bowl. Being from Denmark, I enjoy this on an openfaced coldcut (see Smorgasbord, Traditional Danish Cooking) made from rye bread. Then I add mayonnaise and salt & pepper as needed.