Melt butter over low heat. Stir in leeks and cook until softened. Add zest, squash, and nutmeg; stir for 2 minutes. Add stock; bring to a boil. Simmer over medium heat, covered, until squash is tender, about 15 to 20 minutes. Puree in a food processor or blender (do in two batches). Season with salt and freshly ground pepper.