For the marinade, wrap the garlic cloves (with skin on) in foil and place in an oven at 180°C. Roast for about 30 minutes, until soft. Remove from oven. When cool enough to handle, squeeze out the garlic into a mortar. Add sugar, oregano and spices and pound to form a smooth paste. Combine with vinegar, then place the mixture into a bowl. Whisk in oil and season to taste.
Peel and devein prawns, add to the marinade and leave in a refrigerator for about half an hour.
For the salsa, split and deseed the capsicum. Finely cut into 5mm dice. Place in a large mixing bowl. Dice the onion and jalapeños the same size and add to the bowl with the chopped coriander. Cut the avocado into large dice and add to the bowl with the lime juice. Season to taste and toss gently. For better flavour, let the salsa rest for 10 minutes or longer.
Fire up the barbecue to a medium heat, making sure the grill bars are clean and well-oiled. Remove the prawns from the marinade and drain well. Grill prawns for a minute or two on each side, depending on their size, then place on a plate, covered with foil, to rest for a minute.
Serve the prawns on a platter, drizzled with any resting juices and with the salsa on the side.