Combine squash, lentils, onion, ginger, garlic, curry powder, oil, broth, water and salt in a 6-quart slow cooker.
Cover and cook on low for 6 to 7 hours until squash breaks easily under a spoon and lentils have softened completely.
Turn off the cooker and let the soup stop bubbling for 5 minutes. Blend with an immersion blender, keeping the blade fully submerged, until smooth.
Stir in lemon juice and check that no firm squash pieces remain.
Divide soup among 6 bowls and spoon 2 tablespoons yogurt onto each.
Serve each bowl with one whole-grain roll.