In a bowl, mix together the spring onion, parsley, coriander, and oil. Don't forget to season to taste.
Place half of the cheiro verde in a bowl. Mix in the prawns until coated. Cover and refrigerate for 1 hour to marinate.
Skewer the prawns and preheat the barbecue grill or chargrill to medium heat. Grill the skewers, turning once, until prawns change color, about 3-4 minutes.
Stir the oil and lemon juice into the remaining cheiro verde mixture.
Place the skewers neatly on a serving platter. Arrange the avocado slices on top, then generously drizzle the cheiro verde mixture over them.