Blitz all ingredients together with a stick blender. Transfer to a small saucepan and cook on a low heat until smooth for about 30 minutes, or until the sauce reduces and thickens.
Store in an airtight container in the fridge for about 2 weeks. Suitable for freezing.
If you don't have the time you can just blitz this sauce and not cook it. The sauce won't be quite as rich and thick. Reduce amount of water added to 2 tablespoons if you do this.Because emulisifiers aren't added to this recipe, the sauce can separate slightly. Simply stir to get a smooth texture again.