Preheat oven to 400ºF (200ºC) and place lemon slices, sliced shallot and garlic halves (cut side down) in the bottom of the pan.
Pat the chicken dry, inside and out, to remove any excess moisture. Place the chicken breast-side down on a solid cutting surface then place one of your hands along one side to hold it in place. Using a pair of sharp kitchen shears, remove the chicken’s backbone by cutting along each side straight up the center from top to bottom in two straight lines. Remove the backbone, turn the chicken over so it’s breast-side up and flatten it completely by pressing down firmly with both hands. Season with kosher salt.
Add butter and harissa to a small bowl and stir together with a spatula or wooden spoon.
Rub harissa butter all over the chicken and place it in the baking pan on top of the lemon, garlic and shallot.
Roast the chicken until golden and crispy, about 30-35 minutes, and its internal temperature reaches 165ºF (75ºC) as read on a meat thermometer inserted into the thickest part of the chicken. The harissa will begin to char around the 10 to 25-minute mark. If it starts to look a little too dark for your taste before the chicken is finished cooking, tent with foil for the last 5-10 minutes.
Remove from oven and let rest for 10 minutes before slicing. Serve with pan juices poured over the chicken, together with the lemon, garlic and shallot.