Add the water and sweetener to a small saucepan and heat until the sweetener has dissolved. Set aside to cool.

Roughly chop the pepper (include some of the seeds!) and place it in a small blender, together with garlic paste, salt, and half the sweetened water. Blend until only tiny pieces of pepper are visible.

Add lemon juice, fish sauce, and the remaining water. Blend again and give it a taste test - you may prefer a touch more lemon, or salt, or even fish sauce to get that perfect blend of sweet and sour.

Finally, sprinkle some xanthan gum over the sauce, and give it a final whizz. This will immediately thicken up the sauce - no cooking required.

Transfer the sauce to a bowl, cover, and store in the fridge for up to a week.