Add oil and onion to a large pot and cook over medium/high heat until onions are translucent
Add stock and sweet potato and cook over medium/high heat until sweet potatoes are soft
Add the greens to the pot and cook for about two minutes (do not overcook, you want the greens to stay green!)
Remove from heat, add the cilantro and lemon juice
Add batches of the mixture to a blender or food processor, or use an immersion blender if you have one, and blend until smooth
Add salt and pepper to taste
Optional: Sprinkle with feta cheese