Heat olive oil in a small pan or in your slow cooker insert if designed for stovetop browning. Add onion and garlic and cook 3 minutes or until softened.
Transfer onions and garlic to slow cooker. Add squash, beets, cinnamon, cardamom, allspice, sugar, and salt.
Stir in broth, water, and lemon juice. Cover and cook on LOW for 6 hours or HIGH for 3 hours.
Drain and rinse cashews. Stir into cooked squash mixture. Purée with an immersion blender or in batches with a regular blender until smooth.