Sautee onions in oil over high heat with 1/4 tsp. of salt until beginning to brown, about 5 minutes. Cook over low heat for 25-30 minutes stirring often until they have a deep caramel color.
Put the water and 3/4 tsp. of salt in a pot, add rice and boil for 15 minutes covered.
Trim the white ribs from the chard and coarsley chop the chard and kale.
When the rice has cooked for 15 minutes, stir in the chard and kale. Return to simmer, cover and cook for 10 minutes.
When the onions are carmalized stir a little of the rice liquid into them then add them to the rice 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, about 5 minutes.
Puree the soup with an immersion blender until perfetly smooth.
Stir in the lemon juice. Add more to taste.
