Preheat a grill pan over medium-high heat (or preheat a grill to medium-high).Spray the peaches with oil, then grill them cut sides down, just long enough to make marks, about 2 minutes per side.To serve, place a ball of burrata in the center of each of 4 plates and arrange the grilled peaches, watermelon cubes, and watercress around it.Drizzle everything with the balsamic glaze.