Combine the squash, onion, apple, garlic, olive oil, salt, curry powder, red pepper, cumin and several generous grinds of black pepper in a 6- to 8-quart slow cooker.
Add the water, cover and cook on low until the squash is very tender, 5 to 6 hours.
Add the yogurt and mango chutney and purée the soup using an immersion blender.
Divide among bowls and serve topped with more yogurt, if desired, and pepitas.
