If you have a stovetop, heat a drizzle of oil in a medium pot; add the onions and saute for 4-5 minutes, until soft. Add the garlic and ginger and cook for another minute. Add the curry powder or paste and cook for another minute; then the cilantro.
Add the sweet potatoes, stock, water, lentils and apple; bring to a simmer and cook for 30 minutes, or until everything is very soft. (Alternatively, toss everything in the slow cooker and turn it on low for 8 hours.)
Puree the soup in the pot using a hand-held immersion blender; add the half & half, lime juice and salt, blend again and adjust seasonings. Serve hot.
Serving Size Serves 8.