Finely chop the sweet red pepper, discarding the core and seeds. Cut the tomatoes in half, squeeze out and discard the seeds and juice, and finely chop the flesh. Finely chop the radishes, spring onions and hard-boiled eggs.
Combine the red pepper, tomatoes, radishes, spring onions, eggs, capers and tuna in a large bowl.
Whisk the lemon juice, olive oil, sea salt, pepper, corian