Roughly chop the tomatoes and add them to a saucepan, together with white wine vinegar, onion powder, chipotle powder, and sweetener.

Cook them all together for 10 minutes, stirring occasionally.

Let cool slightly, then add them to a food processor. Blend until smooth, then taste for seasoning, adding salt and pepper as necessary. Sprinkle over a little xanthan gum.

Blend again, transfer to a bottle or jar, and store in the fridge until required.