Preheat the broiler. In a small bowl, combine the vinegar, mustard, garlic and 3 tablespoons olive oil; season with salt and pepper. In a medium bowl, coarsely mash the beans. Stir in the vinaigrette and tomatoes.
Brush the bread on one side with the remaining 2 tablespoons olive oil. Broil oiled side up until golden. Divide the watercress among the crostini and top with the bean mixture and salami ribbons.