Cook the potatoes in gently boiling water for 20 minutes, until tender.Meanwhile, in a salad bowl, mix together the lemon juice, garlic and parsley. Add the red onion and set aside.Drain the cooked potatoes and add them to the salad bowl while they are hot. They will absorb the flavour of the dressing as they cool down.When the potatoes are cool, add the tuna, tomatoes and cucumber. Stir everything together gently and season with black pepper. Serve immediately, or cover and chill to serve later.