How to Make It
Preheat the oven to 325°. In a large bowl, toss the bread cubes with the 2 tablespoons of melted butter, 1/8 teaspoon of the nutmeg and a generous pinch each of salt and pepper. Spread the bread cubes in a single layer on a rimmed baking sheet and bake until golden brown, about 25 minutes. Transfer the croutons to a wire rack.
Meanwhile, in a large saucepan, melt the remaining 4 tablespoons of butter over moderate heat. Add the onions, garlic, 1 tablespoon of salt and 1/2 teaspoon of pepper; cover and cook, stirring occasionally, until the onions are softened, about 8 minutes. Add the celery root, cover and cook, stirring occasionally, until very soft, about 8 minutes. Add the sliced celery, cover and cook until just starting to soften, about 3 minutes. Add the chicken broth and simmer for 10 minutes. Stir in the parsley, spinach and 3/4 cup of the celery leaves and cook for 1 minute, until wilted. Add the cream and simmer over moderately low heat for 10 minutes, until heated through. Season the soup with salt, pepper and the remaining 1/8 teaspoon of nutmeg.
Working in batches, puree the soup in a blender until very smooth. Pour the soup into a clean saucepan and rewarm over low heat, if necessary. Mince the remaining 3/4 cup of celery leaves. Ladle the soup into bowls, garnish with the minced celery leaves and the croutons and serve.