Tuna salad is a cheap, easy, protein-packed no-brainer. I like mine really simple but you can always add extra accoutrements such as relish, mustard, etc. or get really creative with dried cranberries or curry powder. This is just the simple stuff.
Slice off about a sixth of the onion. This is of course an approximation. I don't believe in measuring.
Chop it very finely. A sharp knife is crucial to not crying while doing this. Contacts help too, I've noticed.
Cut the stalk of celery in half, then slice each of them in half lengthwise. Now chop finely.
Throw the onions and celery in a bowl and add the can of tuna, drained. I like to use no salt added tuna a) because mayonnaise is very salty and b) I like to control my seasonings as much as possible. If you're using regular tuna, give it a rinse as well as draining it. Then grind some black pepper into the bowl and toss a glob of mayo on top.
Now use a fork to mix everything together, breaking up the chunks of tuna. I like to add a couple teaspoons water to make the texture creamier. At this point, adjust the seasonings and if needed, add a bit more mayo.
You can now pile a bunch of it on whole-wheat bread, or like I did for lunch, slap a glob of it on top of a simple salad to which i added peanuts, beets, vegetarian bacon bits, oil and vinegar.