Mix together all the marinade ingredients in a small bowl.
Rub the green onions with oil and place on a plate, sprinkle with salt and reserve.
Light your barbeque and let it burn until the charcoal is covered with white ash, then bank the coals to each side of the grill leaving the middle clear, this is where your chicken will sit over indirect heat.
While the barbeque is getting ready, turn the chicken onto its breast, using kitchen scissors cut down the backbone from the parson's nose to the neck. Turn the chicken over so the breast is facing upwards, turn the legs in and with the heal of your hands sharply press down to flatten the breast bones, so it sits nice and flat on the work surface. Push the wings tips up over the breast. You can watch Barney from the BBC Good Food website for tips, on how to spatchcock a chicken.
Smear both sides of the chicken with the marinade. Place it breast side up in the middle of the grill not directly over your banked coals. Cook without turning for about 45 minutes, basting from time to time with any remaining marinade, until the juices run clear when the thickest part is pierced with a knife, the internal temperature is 71-74oC. You will need to add pieces of charcoal to maintain the temperature during cooking.
When it is done, remove to a board that will hold the juices from the bird and cover loosely with foil. Let rest for 20 minutes while you grill the green onions. Place the green onions on the barbeque, turn them as they char and become soft in the middle.
Once the chicken has rested, cut into quarters and pull apart into chunks, place on a serving board with the grilled onions on the side in the middle of the table. Have warmed tortillas and salsa at the ready for everyone to help themselves to make tacos.