Wash spinach in warm, not cold, water to remove the dirt that clings to the leaves.
Melt butter in soup kettle. Saute 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.