Set a wire rack inside a sheet pan.

Pat chicken dry with paper towels and place breast side down on a cutting board. Locate backbone then cut along one side of backbone from front to back. Cut along second side of backbone until it completely releases. Discard backbone or save for homemade stock.

Flip chicken over and press on breastbone to flatten (you should hear it crack). Pat dry again with paper towels then transfer to wire rack, breast side up, and set aside.

Mince the herbs and measure out 2 teaspoons each rosemary, sage and thyme; transfer to a small mixing bowl. Add the softened butter, Better than Bouillon chicken base, pepper, Worcestershire, and salt; mix until thoroughly combined.

Using your hands, gently loosen the skin around the breasts and legs (including thighs and drumsticks). Distribute about two-thirds of the butter mixture underneath the skin, pushing and squishing it around as needed to evenly distribute down the breasts and legs and into the drumsticks. With the remaining one-third of the butter mixture, spread evenly all over the exterior of the skin. *see notes on benefits of prepping ahead of time.

Preheat the oven to 450ºF (232ºC) with the rack set in middle position.

Cut onion in half from root to stem and place halves cut-side down. Trim ends, then thinly slice root to stem into slivers.

Remove the wire rack, with the chicken on top, from the sheet pan and set aside briefly. Add onion slices to the sheet pan and slightly pack them together to create an even layer of onions that will be right underneath the chicken. Avoid spreading them out to the edges of the pan, or they will likely burn. The goal here is for all of the slices to be underneath the chicken and cook in the buttery drippings.

Place the wire rack, with the chicken, back into the sheet pan and transfer to the oven.

Roast the chicken until deeply golden brown and an instant read thermometer inserted into thickest part of breast registers 155-160ºF and thickest part of leg registers 165ºF, 50–60 minutes. If the herbs on skin are starting to burn, lightly tent with foil.

Remove the chicken from the oven and let it rest for at least 15 minutes. Carefully transfer chicken to a cutting board and carve chicken by first removing the breasts, then removing the thighs and wings. Cut the chicken breasts crosswise into ½ inch-thick slices and remove the drumsticks from the thighs. Scrape the onions off the sheetpan, making sure you scrape up any stuck on browned bits, and transfer to a serving platter. Arrange chicken over top and serve.