Combine ground chuck, onion, celery and parsley in a large bowl. Season with salt and pepper. Stir just to blend with a spoon or your hands; do not overwork mixture. Form into 8 patties. Chill until ready to use. Prepare a charcoal fire or set a gas grill to medium-high. Cook burgers, turning once, 8 minutes for medium-rare. Serve on warm toasted buns.