Slice tomatoes and place on baking sheet and place in oven at 170C/375F for 45 minutes to an hour, set aside to cool.Place chillies, garlic, salt, ginger and mustard seeds in a mortar and bash with a pestle. Add a little fish sauce and continue bashing adding more fish sauce til it's well incorporated.Place in a large pot and cook on medium heat for 3 minutes.Add in the tomatoes and the vinegar