To make the balsamic glaze, combine the balsamic vinegar and brown sugar (if using) in a small saucepan. Bring to a boil, and then reduce heat to a simmer.

Cook for 10-12 minutes, stirring frequently to prevent burning, until the glaze is slightly thickened and reduced by half. (If you aren't using brown sugar, it'll take closer to 20 minutes). It should coat the back of the spoon. Pour the glaze into a bowl and let cool to room temperature, at least 15 minutes.

To make the salad, spread the arugula evenly on a medium sized platter. Layer the watermelon cubes, feta cheese, sliced nectarines, and basil over the arugula. Drizzle with 2 Tablespoons (or more) of the balsamic glaze and sprinkle with black pepper and flaky salt.