In a large pot over medium heat add the butter and olive oil. Add the celery, onion, and thyme. Season with salt and pepper, then stir while cooking until the onions and celery are tender. Add the garlic, cinnamon, allspice, and cumin. Stir and cook until fragrant, about 5 minutes. Add the sweet potatoes and apples stirring to coat them with the contents of the pot. Add the vegetable stock and apple cider and then cover. Simmer for 20 to 25 minutes or until tender.
In batches, carefully puree the contents of the pot in a blender or use an immersion blender. Taste and season with salt. Serve warm or chilled.
When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.