Place chicken breast-side down with the neck facing towards you. Using kitchen shears, cut through the ribs on either side of the spine. Open the rib cage and use a heavy knife to score down the sternum. This will help pop out the breast bone and flatten chicken. Season inside the chicken with 1/2 of the teaspoon of salt and the pepper.
Flip the chicken over breast side up and using the palm of your hand, push firmly over the breast bone to force the chicken to lay completely flat.
Marinade
In a small mixing bowl, combine the marinade ingredients, including zest of 1 lemon and 2 tablespoons lemon juice. Cut 3 slices from the other lemon and set aside.
Pour into a ziploc and add the chicken, smooshing the bag to coat the chicken. Let rest in the fridge for a few hours, up to overnight.
Cook the Chicken
Preheat the oven to 425 degrees.
Remove the chicken from the fridge 30 minutes prior to cooking.
Place the whole, raw chicken, cut side down on a cookie sheet. Scoop any marinade onto the top of the chicken and set 3 lemon slices on the bird and slide into the oven.
Bake uncovered at 425˚F for 45 min or until instant read thermometer reads 160˚F when inserted into thickest part of chicken breast. Remove from the oven and rest uncovered for 10 minutes on a cutting board before slicing.