Heat the oil in a medium-large saucepan over medium heat. Add the onion and a pinch of salt and cook, stirring frequently, for 7 to 10 minutes, until the onion is translucent and starting to brown at the edges. Add the garlic and cook for a couple minutes more, until fragrant. Add in the kale, spinach, cilantro, salt, potato, and vegetable broth, and bring to a boil. Stir occasionally as the soup heats up to help the greens wilt down. Once boiling, reduce the heat to low, cover, and allow to simmer for 30 minutes. Puree the soup in batches in a blender (be careful, as the steam can burn), or use an immersion blender to blend the soup right in the pot. Add a generous pinch of cayenne pepper if desired, stir in the lemon juice, and taste. Adjust seasonings as necessary – I like a little extra lemon juice – and serve hot.