Melt the butter in a large pan then add the onion and garlic. Fry for a couple of minutes, just until the onion has softened and then add all the other ingredients.
Cook for around 30 minutes until the mixture has thickened and reduced down, then blitz in a food processor until fairly smooth.
If the sauce is the consistency you like, allow to cool and place in airtight containers until ready to use. If you'd like your sauce a little thicker, pop back on the hob and continue to cook for another ten minutes or so.
If not using immediately, either store in the fridge or freeze. Enjoy!
