Put onion, garlic and butter in a medium saucepan over a medium heat. Cook for 5 minutes or until onion is softened. Season, add chillies and sauce and cook for 3 minutes.
2.
Stir in sugar, Worcestershire sauce, lemon juice and vinegar, then cook for 15 minutes or until liquid is reduced by half. Stir in ketchup, bring to the boil, then pour into sterilised jars and seal while hot. Set aside to cool overnight before serving.