Roughly chop the brown onion and jalapeño pepper, and peel the garlic cloves.
Combine the diced tomatoes (with their juice), chopped onion, jalapeño, garlic and fresh coriander in a food processor.
Add the lime juice, ground cumin, salt and granulated sugar (if using).
Secure the lid and pulse to your desired consistency—pulse a few times for a chunkier sauce or longer for a smoother texture.
Taste and adjust the seasoning, adding more lime juice for brightness, extra jalapeño for heat, or a pinch more salt if needed.
Transfer the sauce to a bowl or a clean jar, cover and chill in the fridge for at least 30 minutes to let the flavours meld.
Serve with tortilla chips, tacos, eggs or grilled meats, or store in an airtight container in the fridge for up to one week.