Drain the tuna and rinse the white beans under cold water. Peel the red onion and slice it paper-thin.
Place the rocket in a large bowl and distribute the beans, tuna, onion and capers over it. The white beans provide slow-release carbohydrates and fibre, while the tuna is a substantial protein source.
Whisk the olive oil with the lemon juice and some black pepper to make a dressing. Pour it over the salad and toss everything lightly. Serve immediately so the rocket stays crisp.