Preheat the oven to 400 degrees. On a half sheet pan or other large shallow roasting pan, lay the onion slices in a single layer. Sprinkle them lightly with salt, pepper and some of the fresh herbs.
Butterfly the chicken by cutting along one side of the backbone. Open the chicken out flat, skin side up. With your palm, firmly press down the breast area to flatten. Place the bird skin-side-up atop the onions. Scatter the carrots, potatoes and garlic around it, along with the rest of the herbs. Sprinkle the olive oil and verjuice or vinegar over everything and finish with a light dusting of salt and pepper.
Roast the chicken about 80 minutes, basting it with pan juices, and turning the vegetables to evenly coat them with the pan juices as well. The dish is done when the center of the breast meat is 170 on an instant-reading thermometer and the vegetables are tender.
If the chicken needs browning, flip on the broiler for a few minutes. If the vegetables need more roasting, remove the chicken, keep it warm and give them another 15 minutes in the oven.
Let the chicken rest 10 minutes at room temperature, then present it on a large platter with its pan juices and the vegetables. The garlic will be almost sweet; enjoy it by squeezing it out of the skins. Taste the garlic with a bite of chicken and vegetables.
Nutrition information per serving: