In a large saucepan cook potato and green onion in 2 tablespoons of the butter over medium heat about about 10 minutes or until potatoes are tender. Slowly add the spinach, one-fifth at a time, stirring just until spinach is limp and dark green after each addition. In a food processor or blender process or blend the spinach mixture, half at a time, until smooth.

In the same saucepan melt remaining 4 tablespoons butter over medium heat. Stir in flour and curry powder; cook and stir for 2 minutes. Slowly add broth, whisking until combined. Stir in spinach mixture and lemon juice. Cook and stir over medium heat until mixture is slightly thickened and bubbly; cook and stir 1 minute more. In a medium bowl stir about 1 cup of the the hot mixture into sour cream. Return mixture to saucepan. Heat through but do not boil. Top with croutons, if desired. Makes 8 servings.