- Preheat your grill to medium heat or your oven to 425ºF.
- Spatchcock the chicken by removing the backbone. To remove the backbone, flip the chicken over so that it sits breast side down and, using a pair of kitchen shears, carefully cut along either side of the backbone. Flip the bird over and press down flatten. Alternatively, ask your butcher to spatchcock (or flatten) your chicken for you.
- Place the backbone of the chicken in a small saucepan and cover with the chicken broth. Add in the yellow onion and thyme and bring to a simmer over medium-low heat, adding more chicken broth to keep everything submerged.
- In a small bowl, mash together the butter, garlic, rosemary, and sage and season well with salt and pepper. Separate the skin from the breast meat and, using a spoon, divide the butter mixture under the skin, distributing it as evenly as you can. Drizzle the oil over both sides of the chicken and season well with salt and pepper.
- If using the oven, transfer the chicken, skin side up, onto a rack lined rimmed baking sheet and roast in the oven for 30 minutes. Turn the heat down to 350ºF and continue to roast the chicken for another 20 minutes or until the internal temperature reaches 165ºF.
- If roasting on the grill, place the chicken skin side up onto the grill, close the lid, and cook for 40 minutes or until the internal temperature reaches 165ºF.
- No matter your cooking method, remove the chicken from the heat, cover it loosely with a piece of aluminum foil, and allow it to rest for ten minutes while you finish the gravy. For the gravy, strain the broth through a fine mesh sieve and transfer back into the pot. In a small bowl, whisk together the cornstarch and cold water or broth then whisk that slurry into the hot broth. Bring the mixture to a boil and cook until thickened. Season with salt and pepper and serve alongside the roast chicken.
For a darker gravy, replace the cornstarch and water with a dark roux. Simply melt 2 tablespoons of unsalted butter over medium heat. Sprinkle over 2 tablespoons of all-purpose flour, whisk it in, and cook over medium, whisking frequently, for two to three minutes or until a deep golden brown. Slowly whisk the warm broth into the roux until fully incorporated and smooth. Season with salt and pepper to taste.