Equipment and preparation: You'll need 4 wooden or metal skewers. If using wooden skewers, soak them for an hour first.
Place the chicken pieces in a clean bowl, then make the marinade. Crush the garlic to a paste with the flat edge of a knife blade, using a pinch of sea salt. Add the garlic paste to the bowl.
Slice the lemon in half and squeeze the juice over the chicken in the bowl. Season with freshly ground black pepper and drizzle over the olive oil. Stir thoroughly. Cover the chicken with cling film and leave to marinate for at least half an hour, but preferably overnight (covered, in the fridge).
Meanwhile, for the romesco-style sauce, prepare the fresh peppers (if using) and the toasted almonds and hazelnuts (if using). Preheat the grill to hot. Cut off the stalk end of the peppers and remove the white ribs and seeds.
Cut the peppers into quarters and place them on a grill rack, skin-side upwards. Place the peppers under the hot grill and grill until the skins are black and blistered, about 10-12 minutes.
Transfer the peppers into a resealable plastic bag and seal the top (the steam created will loosen the skins and make the peppers easier to peel). Allow the peppers to cool. When cool, peel off and discard the skins.
For the hazelnuts, preheat the oven to 200C/400F/Gas 6. Place the hazelnuts onto a baking tray and place into the oven. Toast the nuts for five minutes (set a timer, because they burn quickly), then remove from the oven.
Tip the nuts into a metal sieve and rub them with an oven glove to remove the papery skins. Repeat the process with the almonds. (If using blanched hazelnuts and almonds, just toast them lightly in the oven for 2-3 minutes.)
Heat a frying pan with two tablespoons of the olive oil. Slice two of the garlic cloves and chop the third roughly. Fry the bread until golden-brown on both sides.
While the bread is frying, add the toasted, skinned hazelnuts and almonds to the bowl of a food processor. Tip in the roasted, peeled peppers (or the peppers from a jar, drained), the chopped garlic, the smoked paprika and the cayenne.
When the bread is browned on both sides, remove and drain on kitchen paper. Reduce the heat, then fry the remaining sliced garlic cloves in the same pan. When lightly browned, remove and add to the processor. Break the fried bread into pieces and add to the food processor. Add the red wine vinegar.
Process the ingredients together until finely chopped to a thick paste, scraping down the sides with a spatula as necessary. Keep the motor running and gradually trickle in four tablespoons of water, then the remaining oil. Taste and adjust seasoning. Scrape the sauce into a bowl and cover with cling film until ready to serve.
For the chicken skewers, if using wooden skewers, soak for an hour in cold water (or use metal skewers). Shortly before eating, preheat the grill to hot. Cut the spring onions into 3cm/1¼in lengths, discarding the dark green ends.
Thread chunks of chicken, alternating with lengths of spring onion, onto skewers. Place skewers onto a grill rack and grill for 10-12 minutes, turning once, until chicken is cooked through (the juices will run clear when the meat is pierced in the thickest part with a skewer) and spring onions are patched with brown.
Serve immediately with the romesco-style sauce on the side.
