Slice the red onion thinly and put it in the lemon juice. Leave it for at least 5 minutes while you prepare everything else.
Drain and rinse the cannellini beans under running water, then shake dry.
Drain the tuna thoroughly, pressing out the water, and flake it into a large bowl.
Add the beans, cherry tomatoes, capers and parsley. Pour over the lemon and onion with the olive oil and black pepper.
Fold gently a few times, keeping the tuna in flakes rather than breaking it up. Do not add salt.
Let the salad sit for 5 minutes before serving so the beans absorb the dressing.