Trim leeks of outer layer and stems. Chop white and tender green parts into 1/2-inch chunks (discard tough gray-green parts). Soak leeks in a large bowl of lukewarm water, swishing to dislodge sand. Drain and soak again, then lift leeks, leaving sediment behind.
Melt butter in a heavy-bottomed soup pot over medium heat. Add leeks and season well with salt and pepper. Cook, stirring, until leeks are wilted, about 8 to 10 minutes. Add garlic, cayenne and rice and cook for 1 minute. Add hot broth and bring to a boil, then reduce to a gentle simmer. Simmer until rice is very soft, about 25 minutes. Remove from heat and let cool completely before proceeding.
Using a blender, purée raw spinach with cooled soup mixture. Strain through a fine-meshed sieve, discarding any fibrous solids. Adjust seasoning and add grated nutmeg. Thin with broth or water if necessary. Heat soup before serving, to preserve bright green color. Garnish with a tablespoon of crème fraîche and a sprinkling of chives and tarragon.
Nutrition information per each of 8 servings:
Calories190Fat10 gSodium1,010 mg