Snap off tough ends of asparagus. Remove scales from stalks with a knife or vegetable peeler, if desired. Place asparagus in a large saucepan; add water to cover. Cover asparagus, and cook 10 minutes or until tender. Drain. Cut 2 inches from tip of each asparagus spear; set aside tips and bottoms of asparagus spears.
Coat saucepan with cooking spray; add margarine. Place over medium heat until margarine melts. Add onion, parsley, and coriander; cook, stirring constantly, until onion is tender. Add flour and salt; cook, stirring constantly, 1 minute. Gradually stir in chicken broth; cook, stirring constantly, 5 minutes.
Ladle broth mixture into container of an electric blender; add bottoms of asparagus spears. Cover and process until smooth. Return puree to saucepan, and stir in milk, lemon juice, and pepper. Gently stir in asparagus tips. Cook over medium heat until mixture is thoroughly heated, stirring occasionally. Ladle soup into individual bowls; garnish with lemon slices, if desired.