Preheat your grill to 550°F.
Place the watermelon slices on the hot grate and grill for 3 minutes per side.
Remove the watermelon from the grill and slice each pieces into cubes of about 1½ inch thick.
In a saucepan, combine the balsamic vinegar and honey then bring to a boil for 5 minutes.
In a large salad bowl, combine the arugula salad, crumbled feta cheese, pine nuts, thinly- sliced red onions and grilled watermelon cubes. Mix gently then separate into 4 servings. Add balsamic glaze to taste and enjoy.