Lay salad leaves out on individual serving plates and top equally with strawberries, peaches, cucumber and feta. Drizzle with balsamic reduction (recipe below), if desired.
For the balsamic reduction:
In a small saucepan over medium heat, whisk together balsamic vinegar and brown sugar until sugar has dissolved. Bring to a boil, reduce heat to low, and simmer until glaze is reduced by half, 15-20 minutes. Glaze should coat the back of a spoon. Let cool and pour into a jar with a lid.