Place the tuna in a medium sized bowl. Using a fork break the tuna apart into smaller pieces. I like mine "medium" sized, not all ground up looking.
Now add the onions, olives and celery, mix again.
Then, add the mayonnaise and stir gently to combine. The mixture should be nicely moistened but not mushy.
You can eat it right away but if you want it to taste a lot better, give it at least an hour in the refrigerator in a tightly sealed container. It's even better overnight (I just don't always plan ahead that much).
I really like to serve this in the summer atop a fresh, ripe, tomato. Grocery store tomatoes are dismal so I don't bother unless it is summer. You can also serve it on an avocado half. Of course this will add more calories and about 6 more grams of carbs. Other times I just eat a little out of a bowl.