Step 1Melt the butter in a wide stainless-steel pot (at least a 5-quart capacity) over medium-low heat. Add three-fourths cup of water and the carrot, celery and onion. Increase the heat to bring the mixture to a simmer, then reduce the heat to cook at a low simmer, covered, for 20 minutes.
Step 2Add 4 1/2 cups water and bring to a boil over high heat. Add the spinach and boil for 1 additional minute, stirring until all of the spinach is well-wilted. Do not cover the pot: Volatile acids, which are released when the vegetable is heated, will condense on the lid, fall back into the pot and cause discoloration.
Step 3Puree the entire mixture using an immersion blender, or pour into a blender to puree, and transfer the soup immediately to a hot tureen. Do not strain the soup. Season with 1 1/2 teaspoons salt and a pinch of pepper, or to taste, garnish as desired and serve immediately.