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. (You can also use a regular blender, but be careful puréeing hot ingredients this way; if possible, let it cool a little first and purée in batches.) Divide among bowls and serve topped with more yogurt, if desired, and pepitas.
If you don’t have mango chutney, you can simulate that sweet-tart flavor with the juice of half a lemon and 1 tablespoon of brown sugar instead.
