Over fire or grill, rub a small amount of oil on capsicums (you could use spray oil here) and cook the skin on all sides until charred. If using an oven place oven on maximum temp, place peppers on a wire rack and cook until the skin is blistered. Remove and place in an air-tight container to sweat.
Meanwhile, add the remaining olive oil to a saucepan and bring up to a high heat. Add onion and garlic and a pinch of salt and pepper. Cook stirring occasionally until the onion is soft and translucent.
Reserving any liquid that has formed, remove the skin from capsicums and deseed. Chop the flesh into chunks and add to the onion mix, along with reserved liquid.
Add some more salt and cook until the capsicum is soft. Add vinegar, sugar and cook a further 2 minutes. Remove from heat and set aside. Once cool add capers and herbs and adjust seasoning.