Place the onion, garlic, sweet potatoes, russet potatoes, apples, salt, turmeric and stock in the base of a 6-quart slow cooker, stirring to combine.
Cover and cook on high for 3-4 hours or low for 5-6, or until the vegetables are very tender.
Uncover and add the apple cider vinegar and coconut milk. Using an immersion blender, blend until very smooth.
Alternatively, working in batches, carefully ladle the soup into a blender and blend on high until smooth.
Taste and add additional salt or vinegar if you like.