Prepare the vegetables: peel and cube the butternut squash, peel and quarter the onion, and peel the garlic cloves.
Pour the olive oil into the bottom of the slow cooker, then add the prepared butternut squash, onion, garlic, vegetable broth, coconut milk, cumin, coriander, turmeric, salt, and black pepper. Stir to ensure all ingredients are well combined.
Set the slow cooker on low for 5-6 hours or high for 3-4 hours, cooking until the butternut squash is soft and tender.
Once fully cooked, use an immersion blender to blend the mixture until smooth and creamy. If you prefer, transfer the soup to a countertop blender in batches, blending until smooth.
Taste the soup and adjust seasoning as needed. Serve hot, garnishing with a drizzle of coconut milk or a sprinkle of ground black pepper, if desired.