2 tablespoons mayonnaise
2 tablespoons store-bought or homemade black olive tapenade
2 anchovy filets, minced (about 1 tablespoon)
3 tablespoons extra-virgin olive oil
1 tablespoon finely chopped fresh mint or parsley leaves
4 slices hearty bread
3 ounces watercress
1 teaspoon balsamic vinegar
Kosher salt and freshly ground black pepper
8 to 12 ounces cooked cold lamb leg (see note above)
3 ounces shaved caciocavallo or pecorino cheese
Preheat toaster oven or oven to 500°F. Combine mayonnaise, tapenade, minced anchovies, 1 tablespoon olive oil, and chopped herbs in a small bowl and mix with a fork to combine. Set aside.
Drizzle bread on one side with 1 tablespoon olive oil. Place on a wire rack and toast in toaster oven or oven until warm and just beginning to brown, about 4 minutes. Remove from oven.
Toss watercress with remaining tablespoon olive oil and balsamic vinegar in a medium bowl. Season with salt and pepper.
To construct sandwiches, spread olive/mayo mixture over one side of each slice of bread. Layer lamb, cheese, and dressed watercress on top of two bread slices, then close sandwiches with other two bread slices. Serve immediately.