To spatchcock the chicken, place the bird breast side-down with the neck facing you on a large cutting board. Using kitchen shears, cut down one side of the spine, separating it from the ribs. Then, cut down the other side of the spine, removing it, and saving it for stock or another use. Flip the chicken breast side up and gently press down with your palm on the top of the breast until you hear a snap and the chicken flattens slightly.

Stir together the seasoning mix and the salt and sprinkle it liberally on both sides of the chicken. Transfer the chicken to a large bowl or freezer zip-top bag, cover or seal, and refrigerate for 6 hours to overnight.

Before cooking the chicken, remove it from the refrigerator and place it on a rimmed baking sheet (we love a quarter sheet pan here) breast side-up with the legs pointed towards the sides of the pan. Using your fingers, gently slide them between the skin and the breast meat, then slip two pats of butter under the skin of each breast. Then, let the chicken come to room temperature, about 30 to 45 minutes.

While the chicken is coming to room temperature, preheat the oven to 450°F.

Roast the chicken with the legs pointing towards the back of the oven for 20 minutes. Remove and gently baste the top of the bird and nestle the lemon cut side down on the pan. Return to the oven and roast, basting every 10 minutes, until the internal temperature of the breasts reaches 155°F and the thighs reach at least 170°F. The chicken legs are pretty forgiving, so don't worry if they get higher in temperature, and the chicken breasts will reach 165°F when resting post-roast.

Remove the chicken from the oven and let rest for at least 15 minutes, up to 30 minutes. Carve and transfer to a serving platter, spooning the drippings over the chicken, or return the chicken pieces to the baking sheet and serve straight from the pan.