Remove any of the tough outer lettuce leaves, and reserve them for soup. Wash the rest, spin dry, cut the leaves into ½-inch pieces, and add to the romaine lettuce. Chop the mint leaves into very thin shreds and add to the salad. Sprinkle with salt and pepper, mix, drizzle with oil, and mix again. Add the vinegar and toss the salad. Transfer to a serving platter.