In large pot of boiling water, cook leeks, carrots and celery until tender, about 1 minute. With slotted spoon, transfer to bowl.

Add the pre-soaked (for 15 to 20 min) lentils to cooking liquid; cover, reduce heat and simmer until lentils are tender, about 25 minutes. Drain and add to bowl.

Add parsley, lemon juice, capers, oil, garlic, salt and pepper; toss to coat.

Break up tuna and add to salad, toss and serve.