Melt the butter in a medium sized pot over medium-high heat.Add the onions and salt to pot and cook, stirring often, for 8 minutes, until lightly caramelised and fully softened
Add the remaining ingredients and bring up to the boil.Reduce the heat to medium-low and allow to simmer, uncovered, for 15-20 minutes until nice and thick.
Remove from the heat, allow to cool and then blend until smooth.Season to taste, with salt and black pepper.
Transfer to glass bottles with tight fitting lids.Store in the fridge for 1 - 2 weeks orfreeze up to 3 months.
After defrosting, you may need to boil the ketchup again for a few minutes to re-combine any liquid that has separated.