Preheat oven to 220°C (200°C fan-forced) and line a tray with baking paper.
Pat chicken dry with paper towel. On a secured chopping board, place chicken breast-side down with the tail facing you. Using sharp kitchen shears or chef’s knife, cut down the entire length on either side of the backbone. Cutting through the ribs may require some force; cutting near the pivot point of your scissors, where the two blades are joined, will generate more force than snipping with the tips. Once cut, remove backbone. Note: To maintain medium heat (170°C to 190°C), keep lid closed and adjust lit burner as necessary. Directions were developed using a gas grill. Grills vary; cooking time is approximate.
Turn the chicken over so that the breast-side is facing upward. Press on the breastbone firmly with the palms of your hands to flatten. Aim to have the breasts and thighs at the same level; you might hear some crunching.
Brush chicken evenly with mustard. In a bowl, combine Mixed Herbs, Garlic Granules, Black Pepper and sea salt flakes. Remove 1 tablespoon of seasoning blend and set aside. Use the remaining seasoning mix to rub all over the chicken before transferring to a large aluminium pan or glass dish. Cover with cling wrap and refrigerate for at least 30-60 minutes before roasting.
Meanwhile, in a medium bowl, toss potatoes with oil and remaining 1 tablespoon of seasoning blend until well coated. Place on prepared tray along with the lemon halves. Place marinated chicken on top and roast in the oven for 45 minutes.
After 45 minutes of cook time, remove tray from oven and baste with melted butter. Return tray to oven and increase temperature to 240°C (220°C fan-forced). Cook for another 15 minutes or until skin is golden and chicken is cooked through (internal temperature is 70°C). Once cooked, remove from oven and baste chicken again with remaining melted butter.
Squeeze lemon from tray over top before slicing chicken and serve with potatoes.