Dice the potatoes while still warm and combine with one tablespoon of the lemon juice, the vinegar, salad oil, salt and pepper. Let the potatoes marinate in the refrigerator for at least one hour.
Tear the lettuce and romaine into bite-size pieces and place in a salad bowl along with the scallions, celery, cucumber and marinated potatoes. Mix the remaining lemon juice with the mayonnaise. Add to the salad and toss tightly. Garnish with tomato slices or wedges.