Drain the red kidney beans using a colander. Peel the onion and cut into rings. Wash the cucumber and halve lengthwise. Deseed with a spoon and slice the cucumber.

Wash and dry the tomatoes. Halve them, remove the green bits around the stem and slice the tomatoes. Drain the capers and the tuna.

Mix the sherry vinegar and garlic oil and season with salt and pepper.

Wash the lamb's lettuce. Transfer the lamb's lettuce, beans, onions, cucumber, tomatoes, capers and tuna into a bowl. Pour over the salad dressing and leave to steep for approximately 5 minutes.

Arrange on plates and serve.