In a bowl, add chopped red onion and fresh lime juice. Mix well and set aside for 5 minutes.
Add the chopped tomatoes, chilli (if using), fresh coriander and sea salt. Mix well, cover and set aside in the fridge for 1 hour. Bring back to room temperature before serving with freshly fried tortilla chips.
This salsa tastes best the same day but will also keep well in the fridge for 2 days.