Ingredients
120g red onion, finely chopped 
200g tomatoes, chopped small 
1-2 green birds eye chillies, finely chopped depending on how spicy you like your food 
1 tbsp Olive oil 
1 tbsp Balsamic vinegar 
1 tsp sugar

Method
Put everything in a non-metallic bowl and stir. Check the seasoning and sourness, you want a bite but it should not be too acidic.