1. Heat oil in large skillet over high heat. Add onions and 1/4 tsp. salt; cook, stirring until onions begin to brown, about 5 minutes. Reduce heat to low, add 2 T water and cover. Cook stirring frequently, until onions are greatly reduced and have a caramel color, 25 minutes.
2. Meanwhile, combine 3 cups water, remaining 3/4 tsp. salt, and rice in soup pot or dutch oven. Bring to a boil. Reduce heat to maintain a simmer, cover, and cook 15 minutes. Trim ribs and tough stems from the greens and spinach; discard. Coarsely chop greens and spinach.
3. When rice has cooked 15 minutes, stir in greens. Return to a simmer; cover and cook 10 minutes. When onions are caramelized, stir a little greens simmering liquid into them; immediately add them to rice along with spinach, broth and cayenne. Return to a simmer, cover, and cook until spinach is tender but still bright green, about 5 minutes more.
4. Puree soup in pot with immersion blender until perfectly smooth, or in regular blender in batches. Stir in lemon juice and garnish with olive oil.
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