Preheat oven to 400ºF (200ºC) and place lemon slices, shallot and garlic cloves in the bottom of the pan.
Add butter, harissa, salt and pepper to a small bowl and stir together with a small spatula or wooden spoon.
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.
Rub harissa butter all over the chicken and place it in the baking pan on top of the lemon, shallot and garlic.
Roast the chicken until golden and crispy, about 55-60 minutes, and its internal temperature reaches 165ºF (75ºC) as read on a meat thermometer inserted into the thickest part of the chicken. If you should find the surface of the chicken to be a little dry, baste it with some of the pan juices at the 45-minute mark.
Remove from oven and let rest for 15 minutes before slicing. Serve with pan juices poured over the chicken, together with the lemon, garlic and shallot.