In a medium bowl, roughly mash half the chickpeas with a fork, leaving the rest whole.
Add the tuna and break it up with the fork.
In a small bowl, whisk together mayo, lemon juice, and Dijon mustard.
Add the dressing to the tuna mixture along with the diced onion.
Season with salt and pepper, then fold in parsley if using.
Taste and adjust seasoning as needed.
