Heat the oil in a large frying pan over medium heat.
Sauté the onion for two to three minutes until it begins to soften.
Add the sliced peppers and cook for ten minutes, stirring occasionally, until just softened.
Add the garlic, capers (if using) and vinegar.
Simmer gently for a few minutes, stirring occasionally.
Taste carefully (it will be very hot!) and add salt and pepper to taste.
Transfer the mixture to a serving dish and allow to cool for at least 5 minutes before serving. You can also cool completely to room temperature. Either way, garnish with the chopped parsley just before serving.