Heat 3 tbsp plant butter in large pot over medium. Once melted, add onions and 1 tsp salt, coating well in butter, sautéing until lightly caramelized, about 8 min.
Add 1 tsp black pepper, toast for 1 min.
Add sweet potatoes + apple, cooking 10 min.
Add spices, ginger and garlic, sauté for 1 min or until fragrant.
Add broth + cashew milk. Bring just to a boil, then reduce to low and cover. Cook about 40 min, until potatoes are tender.
Remove from heat. Using hand-held blender or stand alone, blend soup until smooth. Add more broth if necessary to loosen up texture.
Add soup back to large pot, add 1 tsp Apple Cider Vinegar, and return to low heat. Let it heat on low a few min. Serve.