To make the sauce, whisk together the water, vinegar, ketchup, and simple syrup together in a jug or high-sided bowl. Taste for flavor and add more syrup, ketchup, and/or vinegar if desired. 

Sprinkle the xanthan gum over the top, and use an immersion blender (stick blender) to thoroughly combine. Set aside.

Next, cook the cauliflower until only just tender. You could use steam-in-the-bag microwave bags, or boil the cauliflower in a pan. Either way, drain thoroughly, then add the cauliflower florets to a large dry skillet.

Place over a medium heat and season with salt and pepper. Steam the cauliflower in the skillet for 5-10 minutes to remove any excess liquid, stirring occasionally. 

Add the prepared sweet and sour sauce to the skillet, and stir well to combine. Continue to cook for another five minutes.

Transfer the cauliflower to a serving bowl and garnish with the green onions.