Wash and dry the watercress and lettuce. Cut lettuce into bite size pieces and arrange on 4 salad plates. Place watercress on top of lettuce in the center of the plate. Cut the avocados in half and slice 4 (1/2-inch) thick slices. Fan over the watercress. Whisk vinegar, olive oil, and lemon pepper together. Drizzle mixture evenly over the salad and serve.