Peel and finely slice the onions, roughly chop the tomatoes and deseed and finely slice the chilli.
Put everything in a pan, season to taste and stir well to combine. Simmer for 30 to 40 minutes or until jammy. Pour into a sterilised jar and leave to cool. Keeps for up to 4 weeks in the fridge. Perfect on toast with a chunk of cheddar.