You can grill the chicken ahead of time, and store, refrigerated, until serving.
Rinse the chicken under cold water, and pat dry with paper towels. Transfer chicken to a large shallow baking dish. Add the garlic, the wine, and the olive oil. Toss to combine, and cover dish with plastic wrap. Marinate, refrigerated, 1 hour to overnight.
Heat grill until medium hot. Remove chicken from marinade. Discard marinade. Sprinkle chicken with the salt and pepper, and arrange pieces on grill. Cook until chicken is dark golden-brown on bottom, 15 to 20 minutes. Turn pieces over, and cook until chicken is cooked through and juices run clear when pierced with a sharp knife, 15 to 20 minutes more.
Place onion and lemon rounds on the cooler area of the grill; cook until soft and golden, about 5 minutes. Transfer chicken, lemon, and onions to a serving platter. Garnish with parsley, and serve.