Measure the balsamic vinegar and pour into a saucepan. Bring it to a gentle boil over a low heat. Cook it until the balsamic vinegar has reduced to a nice thick glaze (but still pourable), about 15 minutes. Allow the reduction to cool to room temperature before you serve it.

When you are ready to assemble the salad, cut the tomatoes into thick slices. Arrange them on a platter, alternating them with the mozzarella slices. Tuck the whole basil leaves in between the tomato and cheese slices.

Drizzle on the balsamic reduction. Then drizzle olive oil in a thin stream over the top. Finally, sprinkle on salt.