Place the red onion in a small bowl and cover with cold water. Let sit while you prepare the rest of the salad (this preserves its flavor but takes off some of the harshness...and the continual red onion aftertaste).
To a large mixing bowl, add the chickpeas, tomatoes, cucumber, onions, and bell pepper. Drain the tuna and flake into the bowl. Add the arugula.
Prepare the dressing: In a small bowl or measuring cup, stir together the lemon juice, olive oil, garlic, salt, and pepper. (You can also shake them all together in a mason jar with a tight-fitting lid). Drizzle enough over the salad to moisten it, then toss to coat. Sprinkle the feta and parsley over the top, then toss lightly again. Taste and add additional salt, pepper, or dressing as desired. Enjoy!
