Drain the white beans well and rinse them under cold water. Cut the red onion into thin half-rings and halve the cherry tomatoes.
Put the arugula in a large bowl and fold in the beans, onion and tomatoes. Drain the tuna briefly and crumble it into large chunks over the salad.
Whisk the olive oil with the lemon juice, salt and pepper to make a simple dressing and drizzle it over everything. Toss the salad gently so the tuna doesn't fall completely apart and adjust the seasoning with extra pepper if needed.