Melt the butter or coconut oil over medium heat. Add the onions, and sweat without browning (adjust heat as needed) for about 10 minutes. Add 1 tsp salt.
Add in the barbecue sauce, and stir to coat.
Add in the carrots and sweet potatoes, and allow to cook together without browning for about 5 minutes, and the pour in the coconut milk or stock, the cider and the vinegar. Bring to a simmer and allow the potatoes and carrots to soften, about 15-18 minutes.
Purée using an immersion blender, a food mill, or in a Vitamix (be careful not to speed it up too high or it will whip a lot of air in your soup and it will become very frothy), and make totally smooth.
Taste to adjust seasoning, add a pinch more salt and the pepper if needed.
Serve with your choice of garnish(es).