To make the pesto, whizz all the ingredients together in a food processor with 5 tbsp water to make a paste. Add a little seasoning to taste.
Toss the prawns with the chilli, garlic and olive oil. Strip most of the leaves from the rosemary twigs, leaving the top few in place. Cut the base of each twig diagonally to make a sharp point and spear 3 prawns onto each twig, until all the prawns are used up.
Finely chop a tablespoon of the reserved rosemary leaves and sprinkle over the prawns. Season well and add to the barbecue or pan and cook for 2-3 minutes on each side, or until cooked through. Serve the prawn skewers with the pea pesto and some lightly charred lemon wedges.