Remove any giblets from inside the bird.
You can add these to the gravy you make later, if you enjoy giblets in your gravy.
Use a pair of heavy-duty kitchen shears or a heavy knife to cut out the 1” strip of backbone
Cutting along both sides of the backbone from the bottom of the bird all the way up through the neck.
Salt both sides of the bird and allow chicken to rest at room temperature.
Preheat oven to 400F.
Use 4 Tbsp butter to cook the shallots on medium heat until translucent.
Add the chopped mushrooms and cook for an additional 5 minutes.
Add the chopped parsley and cook for an additional 10 minutes.
Season with salt and black pepper to taste.
Allow the mixture to cool slightly.
Carefully use your fingers to separate the chicken skin over the breasts and the thighs from the meat.
Carefully stuff the shallot and mushroom mixture in between the skin and meat.
Place the chopped onion and chopped carrot in a roasting pan with the rosemary sprigs
Gently drizzle with vegetable oil.
Place the stuffed chicken skin-side-up on top of the vegetables and rosemary.
Spread the remaining 4 Tbsp softened butter all over the outside of the chicken.
Bake at 400F for 30 minutes.
Lover heat to 350F, and continue to bake for an additional 40-60 minutes, or until the meat in the thickest part of the thigh registers 160-162F.
Remove the chicken from the oven and allow to rest at room temperature for an additional 15 minutes while you make the gravy.
To make the gravy, remove the onions and carrots and rosemary from the roasting pan.
Pour the chicken stock into the roasting pan and scrape off any browned bits that may have gotten stuck to the bottom of the pan.
If you are adding giblets to your gravy, chop them up into a fine dice and add them to the liquid now.
Bring to a simmer on the stove.
Using a whisk, slowly add the Wondra flour to the simmering liquid until you have reached your desired gravy consistency.
If your gravy is too thin, slowly whisk in a little more Wondra flour.
Season with salt and freshly-ground black pepper to taste.
Add a tiny splash of sherry vinegar (about 1/4 tsp) to the gravy.
Slice chicken and place on plates with the roasted onions and carrots.
Serve with hot gravy and mashed potatoes.