Prepare the Dry Brine: In a small bowl, mix together the coarse sea salt, brown sugar, and black pepper.

Brine the Chicken: Unwrap your chicken and remove any giblets. Pat the chicken thoroughly dry, inside and out, with paper towels. Sprinkle the brine mixture evenly all over the chicken, including inside the cavity. Place the chicken on a wire rack set inside a baking tray and refrigerate, uncovered, for at least 2 hours, or ideally overnight (up to 24 hours). This step is vital for drying out the skin.

Make the Herb Butter: In another small bowl, mash the softened butter with the minced garlic, chopped rosemary, and thyme until well combined.

Prep the Chicken: Remove the chicken from the fridge. Gently rinse off the excess brine under cold water and then pat it completely dry again with paper towels. This is a critical step for crispy skin. Carefully slide your fingers between the skin and the breast meat to create a pocket. Push about two-thirds of the herb butter into this pocket, spreading it evenly over the breast meat. Rub the remaining butter all over the outside of the chicken.

Truss the Chicken: Place the quartered onion and halved lemon inside the chicken's cavity. Truss the chicken securely with butcher's twine to keep the wings and legs close to the body. What works best for me is to ensure it's a tight, compact parcel; this helps it rotate evenly and cook through without any parts burning.

Set up the Rotisserie: Carefully slide the trussed chicken onto the rotisserie spit, securing it with the forks. Make sure it's centred and balanced. Prepare your barbecue for indirect cooking at around 180°C (350°F). For a gas barbecue, this usually means leaving the burners directly under the chicken off and using the ones on the sides.

Cook the Chicken: Place the spit onto the rotisserie motor, place a drip pan with a little water underneath to catch the fat, and turn it on. Close the lid and let the chicken cook for approximately 1 hour and 20-30 minutes.

Check for Doneness: The chicken is cooked when the juices run clear and an instant-read thermometer inserted into the thickest part of the thigh (avoiding the bone) reads 74°C. For confirmation on safe cooking temperatures, you can check the guidance from the Food Standards Agency.

Rest is Best: Carefully remove the hot spit from the barbecue. Let the chicken rest on the spit for a few minutes before sliding it off onto a carving board. Tent loosely with foil and let it rest for a full 15-20 minutes. This allows the juices to redistribute throughout the meat, guaranteeing a moist result. Carve and serve immediately.