Transfer the reserved potato mixture to an ovenproof sauté pan or casserole and reheat in the oven. Add the lettuce and tomatoes to the pan and let wilt in the oven for 1 to 2 minutes.
To make the dressing or marinade whisk together the red wine vinegar, olive oil, parsley, chives, garlic, salt and pepper. Place the tuna in a shallow dish and pour over half of the dressing.