Melt butter in soup kettle. Sauté carrot, spinach, and onion until soft. Sprinkle in flour and stir over low heat until blended. Slowly add stock, stirring until smooth. Heat to a boil, reduce heat, add parsley and simmer, uncovered, 5 minutes. Press mixture through food mill. Add lemon juice and heat thoroughly. Season with salt and pepper. Garnish with croutons.