Combine balsamic vinegar and honey in a small saucepan over medium heat until just boiling. Reduce heat to low (a slow simmer is what you’re after), stir occasionally, and let simmer until reduced by at least half. The balsamic will continue to thicken as it cools. Once completely cool, store in the refrigerator. I like to keep mine in a little plastic squeeze bottle.
Drizzle arugula with a little bit of olive oil and season with Kosher salt. Top with cubed watermelon, pickled red onions, feta cheese, and pine nuts. Drizzle generously with the balsamic reduction. Devour immediately.