Heat oven to 450 degrees F. In small bowl, combine parsley, chopped sage, garlic and chopped thyme. Pat chicken dry with paper towels and place in a large roasting pan. Loosen skin around breast, thighs and legs and tuck half the herb mixture (about 2 tablespoons) under the skin. Rub 1/4 teaspoon of the salt under the skin.
Cut one of the onions in quarters. Place two quarters in the cavity of the bird, along with the sage and thyme sprigs. Tie legs together.
Transfer roasting pan to oven. Roast at 450 degrees F for 30 minutes. Meanwhile, halve or quarter potatoes. Quarter the second onion, then cut all onion pieces into 1-inch chunks. Combine potatoes, onion, remaining herb mixture, the oil, pepper and remaining 1/4 teaspoon salt in a bowl. Toss to combine.
After 30 minutes, reduce oven temperature to 400 degrees F and add potato mixture to pan. Roast at 400 degrees F for 55 minutes or until instant-read thermometer inserted in thickest part of the thigh registers 180 degrees F. Let chicken rest 5 minutes before removing skin and carving.