Whisk ingredients in a small bowl.

Season with salt and pepper, to taste. Set aside.

Place eggs into a medium saucepan. Cover with cold water and bring to the boil. Cook for 5 minutes (semi-firm yolks). Remove eggs and immerse in cold water until completely cool. Peel and roughly chop.

Combine celery, onions, cucumber and capers with half the dressing in a large bowl. Gently fold in tuna, boiled eggs and parsley.

Separate lettuce leaves and arrange on a serving plate. Spoon in egg salad. Drizzle with remaining dressing.