To make prawn marinade, combine all ingredients in a bowl. Add prawns and rub marinade all over. Season. Refrigerate for 30 minutes or up to overnight.
To make spicy walnut pesto, place whole capsicums and chilli on an oven tray and cook under a hot grill (or on a barbecue), turning until skin is charred and blistered. Place in a bowl and cover with plastic wrap. Stand for 10 minutes.
Peel away and discard skin. Chop flesh coarsely and place in a processor. Process to form a puree. Add remaining ingredients. Process to form a chunky puree.
Heat a barbecue or chargrill over high heat. Cook prawns, turning, until charred and almost cooked through.
Serve prawns with pesto and lime wedges. Garnish with micro herbs.