Salsa Criolla probably is the most important and most popular side dish / sauce in Peru. Prepared with red onions, aji amarillo, the juice of Peruvian limes and some freshly chopped coriander or parsley, Salsa Criolla accompanies many typical local dishes and just has to be on the table when enjoying a great Peruvian meal.
Peru's popular onion–chili sauce is quick and easy to prepare at home. But as it gets its authentic flavor, at least partly from the aji amarillo, you should really use this kind of chili and don't substitute it. Nowadays you can find aji amarillo sometimes fresh and always in glasses or frozen at many Latin American Food Stores, even online.