Wash hands with soap and water.
Prepare the dressing first. In a small pan over medium heat, whisk together balsamic vinegar and brown sugar. Bring to a low boil, reduce the heat to medium-low and simmer, stirring occasionally.
Let the mixture thicken and reduce to ½ cup for about 8 to 10 minutes.* Once the mixture is thick enough to coat the back of a spoon, remove from heat and let cool for about 15 minutes.
While the dressing cools, prepare the salad.
Wash hands with soap and water.
Scrub watermelon, cucumber and onion with a clean vegetable brush under running water.