1. Remove the ribs from the kale and discard. Coarsely chop the leaves.
2. Heat oil in a large skillet over medium low heat and cook the onions, stirring occasionally, until they’re caramelized and golden brown.
3. While the onions are cooking, combine 3 cups water and ¾ teaspoon salt in large pot or Dutch oven. Add rice and bring the water to a boil. Reduce heat to maintain a simmer, cover, and cook for 15 minutes.
4. When the rice has cooked for 15 minutes, stir in the kale. Return to a simmer, cover, and cook for 10 minutes.
5. Add the caramelized onions to the rice pot along with the spinach, broth, and cayenne. Return to a simmer, cover and cook, stirring once, until the spinach is tender but still bright green.
6. Blend the soup until it’s smooth with an immersion blender or transfer it in batches to a high-speed blender.
7. Taste and add more lemon juice, salt, or cayenne pepper, if desired. Garnish each bowl of soup with a swirl of extra virgin olive oil.