For the aioli:
For each sandwich, cut one 1/2-inch slice of bread from the loaf. Generously spread aioli on one side of the bread slice. Lay face-up on serving plate. Cut 1/4-inch slices from the tomato and shingle three tomato slices on top of bread. Sprinkle with kosher salt. Top tomato layer with a small handful of watercress. Drizzle with olive oil and balsamic vinegar.