Preheat oven to 375 degrees. Spray a large nonstick roasting pan with non-stick cooking spray.
Cut 4 of the onions into quarters. In a medium bowl, toss the onion quarters, carrots and potatoes with 1/2 cup of the salad dressing (use more if not fully coated) and place in roasting pan.
Slice the remaining onion and place the slices inside the chicken (after removing giblets from the cavity of the chicken) with the 6 sprigs of parsley.
Place the chicken, breast side up, on top of the vegetables in the roasting pan. Brush the chicken with 1 T. of the salad dressing and sprinkle it with salt and black pepper.
Bake the chicken in the preheated oven for about 1-1/2 hours, or until an instant-read meat thermometer registers 180 degrees. While the chicken is cooking, brush it with the remaining salad dressing and pan drippings every 30 minutes.
Remove the onion and parsley from inside the chicken and discard.
To serve, carve the chicken and sprinkle the vegetables with salt, black pepper, and the chopped parsley.