Preheat grill to medium-high. In a small bowl, whisk together the vinegar, mustard, salt, sugar, and pepper until blended and smooth. Pour 2 tablespoons of the dressing into a cup and brush over the chicken and onion wedges. Let the chicken and onion stand for 5 minutes. Grill the chicken for 12 to 15 minutes, or until a thermometer inserted in the thickest portion registers 160° and the juices run clear, turning once. Grill the onion for 3 to 5 minutes on each side, or until soft. In a large bowl, toss together the salad greens, grapes, basil, and the remaining dressing until evenly coated; transfer to a platter. Cut the chicken into 1/2-inch-thick slices and arrange over the salad.
Produce for Better Health Foundation.