Zest and keep aside the lemon zest for another use. Juice half of the lemon and cut the other half into 4 pieces then thinly slice.
Bash all the rosemary leaves into the mortar and pestle together with the salt and peppercorns. Peel and add the garlic, the cayenne peppers that are cut into thin slices (cayenne pepper is optional to add if you want some kick), muddle, and bash again until all are broken down and the intense aroma is released. Add the lemon juice, lemon slices, and sugar, stir to combine. Taste and add more seasoning if needed. Another option if you don’t have mortar and pestle, crush the garlic under the flat side of a big knife, and firmly press with the heel of your hand. Add the cayenne peppers that are cut into thin slices (cayenne pepper is optional to add if you want some kick), salt and pepper, and keep bashing and muddling with the side of your knife until the aroma is released. Add to the lemon juice, lemon slices, and sugar. Stir to combine.
Using a Ziploc bag or a nonmetal container, put the chicken and massage gently with the marinade. Seal the bag or cover tightly the container and put it in the fridge. Let it marinate for at least 4 hours or better overnight.
Take out the chicken from the fridge 1 hour before cooking. Remove the marinade and discard it. Pick out the bits and pieces of garlic and rosemary that is stuck in the chicken. Rub 1 tbsp of vegetable oil to each chicken half.
Set grill for both direct and indirect grilling and fire up your grill to 400-450°F.Grill the chicken, skin side down for 5 minutes, or until the skin is lightly charred. Flip it over and grill for another 3-5 minutes. Transfer the chicken to the indirect grill, close the lid and continue roast-grilling for 10-15 minutes or until fully cooked.
Slice the chicken into 1-inch strips and serve with any vegetables of your choice and our family favorite, oven-roasted rosemary-garlic potatoes (recipe follows)