Step 1 Wash the spinach very well in several changes of water. Spinach tends to hold dirt, so make sure you wash it until the water runs clear. Drain and shake the spinach dry.
Step 2 Place a large pan (big enough to hold the spinach) over medium high heat. Add the spinach and cook until it just wilts (the water clinging to the leaves after washing generates enough steam for cooking), about 2 minutes. Immediately remove from heat. Drain the spinach in a colander, squeezing out any excess moisture, and set aside.
Step 3Rinse and dry the pan. Add the butter and melt gently over low heat until softly foaming, then add the shallots and sweat for 5 minutes, or until softened and translucent. Add the garlic and cook for a minute or two, then season generously with salt and pepper.
Step 4Add the spinach and turn once or twice to combine, then pour in the stock and turn up the heat to high. Bring to a simmer, then immediately remove from the heat. Don't overcook it or the spinach will turn color. Coarsely puree the soup using an immersion blender, or using a stand blender, in batches as necessary. Don't puree it too fine or the soup will seem watery.
Step 5Return the soup to the pan and stir in the creme fraiche. Add the grated nutmeg, then check for seasoning -- adding a little more nutmeg, pepper and/or salt to taste. Salt in particular is key to the flavor of this soup. Reheat gently and sprinkle with grated lemon zest just before serving. This makes a generous 5 cups of soup.