1.For the caper dressing, combine all of the ingredients in a screw-top jar and shake well.
2.To butterfly the sardines, cut through the underside of the fish to the tail. Break the backbone at the tail and peel away the backbone. Trim sardines.
3.Cook the sardines on a heated, oiled barbecue (or grill or pan-fry), in batches, until browned on both sides and just cooked through. Serve with tomatoes and onion, spoon over caper dressing and top with parsley leaves.
Have the sardines cleaned and the heads removed at the fish shop. The fishmonger may also butterfly them for you. Best made close to serving. Not suitable to freeze. Not suitable to microwave.