Heat olive oil in a large skillet over medium heat. Add onion and cook until softened and just starting to turn golden at the edges, about 6-8 minutes. Add garlic and cook 1 minute more until fragrant. This step builds a sweet, deep base that raw onions in the slow cooker cannot match.

Transfer the onion mixture to your slow cooker. Add cubed butternut squash, broth, maple syrup, cinnamon, nutmeg, salt, and pepper. Stir to combine. The squash should be mostly submerged; add a splash more broth if needed.

Cover and cook on LOW for 6-7 hours or HIGH for 3-4 hours, until the squash is completely tender and mashes easily against the side of the pot with a spoon. The low setting yields slightly sweeter, more developed flavor.

Stir in coconut milk. Using an immersion blender, puree the soup directly in the slow cooker until completely smooth and velvety, about 2-3 minutes. Alternatively, blend in batches in a standard blender, filling only halfway each time and venting the lid to prevent explosions.

Taste and adjust salt as needed - underseasoned squash soup tastes flat. Ladle into bowls, drizzle with additional coconut milk, and scatter toasted pepitas over top. Add fried sage leaves if using. Serve hot with crusty bread.