Place red onion in cold water while prepping the rest.
Add chickpeas, tomatoes, cucumber, onions, and bell pepper to a large bowl.
Drain and flake tuna, then add to bowl with vegetables.
Whisk together lemon juice, olive oil, garlic, salt, and pepper for dressing.
Drizzle dressing over salad and toss lightly.
Add arugula, parsley, and feta, then toss again before serving.
