Stir together the mayonnaise, pesto and chopped walnuts in a small bowl. Spread evenly over all the slices of sourdough bread.
Thinly slice the cheese and divide the slices equally among 6 slices of bread. Top with the turkey, apple and watercress, dividing equally. Cover each with one of the remaining slices of bread. Refrigerate the sandwiches if not serving immediately. Makes 6 sandwiches.