Put sweet potatoes, apples, onion, and garlic into the crockpot. Sprinkle sage, cinnamon, nutmeg, salt, and pepper over top.
Pour in broth and apple juice. Stir to combine so spices are evenly distributed.
Cover and cook on low for 6–7 hours or high for 3–4 hours, until sweet potatoes are very tender.
Use an immersion blender to puree until smooth, or transfer in batches to a blender and return to pot.
Taste and adjust salt and pepper. Stir in a splash of water or broth if too thick.
Serve with a dollop of Greek yogurt or sour cream if you like.