Add all of the ingredients to a medium mixing bowl and stir until evenly combined. Be sure to break up the tuna as you stir if needed.
Serve immediately or let sit for up to 20 minutes for the flavors to marry together.
We like this tuna salad on bread with lettuce, with crackers, or over tomato slices for a low-carb option.
Notes
I find that everyone likes their tuna salad a little differently when it comes to the amount of mayo used. For my family, 1 cup of mayo for 4 cans of tuna is perfect. You can start by adding 1/2 cup of mayo, mix everything up, and then see if you like the consistency of the salad. If you can it looser, add more mayo until your desired consistency is reached.
If you let the tuna salad rest, the moisture from the red onion and celery will pull out into the mayo some. This can cause the tuna salad to be on the wet side. You can start with less mayo, and then mix more in later if you think it's needed.