To make the rub, just mix it all together and it’s ready to use!

Spatchcock the chicken: Put it breast-side down on a board and, using a pair of poultry scissors, cut out the backbone. Turn the bird over and press down gently on the breast bone, then splay the chicken open.

Season with salt on both sides. Make a paste with the olive oil and the BBQ rub and coat the bird all over with it. Leave to marinate for 3-4 hours or overnight.

Place the bird skin-side down on a barbecue grill and cook over a medium heat until tender and cooked through. Turn once and move to a lower heat if necessary to prevent scorching. If using a thermometer to check the temperature, pull the bird off the heat at 65ºC/150ºF. Leave to rest for 15-20 minutes.

Meanwhile, make the sauce: put all the ingredients in a pan and bring to the boil, stirring constantly with a whisk.

Toss the spring onions in enough vegetable oil to coat, then place on the barbecue and cook until lightly charred.

Carve off the chicken legs and thighs, then carve the white meat from the bone. Brush the sauce over the meat. Garnish with the grilled spring onions and serve with the rest of the sauce on the side.

"You’re going to fall in love with this Carolina-style sauce. I really like its tanginess with the chargrilled chicken skin. It also stands up well to the charred spring onions, which make the perfect garnish for this dish. Keep any leftover mustard sauce in the fridge -- it’s great with sausages." -- Brad McDonald, author of Deep South: New Southern Cooking