In a small bowl, combine walnuts, blue cheese, butter, and port (if desired). In a separate, medium-size bowl, toss apple slices with lemon juice and olive oil.
Spread about 1 tablespoon of the blue cheese mixture on bread. Arrange 3 or 4 apple slices on top. Arrange a few pieces of watercress on top of apples. Place a second piece of bread on top and gently flatten. Trim crusts; then cut sandwiches diagonally twice, for 4 triangles. Repeat with remaining bread slices.