Heat oil in a saute pan. Add the onions, leeks, thyme, salt and pepper. Cook over medium heat for 10 minutes (until the onions are translucent). Add the Garlic and cook for 2 more minutes. Add the drained lentils, celery, carrots, chicken stock and tomato paste. Cover and simmer on low heat for 20 minutes. Add the vinegar and season to taste.