Heat the oil in a saucepan over a high heat. Reduce the heat to medium, add the celery and onions and fry, stirring, for 2 minutes. Add the garlic and fry for a further 1–3 minutes until the onions are softened.
Add all of the remaining ingredients and stir until the sugar and molasses dissolve. Season with salt and pepper, then bring to the boil, stirring. Pour the sauce into the slow