In a saucepan, combine the chicken broth, chopped onion, asparagus, lemon juice and ground mace. Bring mixture to boiling. Reduce heat; cover and simmer for 8 minutes or until vegetable is tender.
Place half of the vegetable mixture in a blender or food processor. Cover and blend 30 to 60 seconds or till smooth. Take care when blending hot liquids. Pour into bowl. Repeat with remaining vegetable mixture; set all aside.
In the same saucepan, melt the butter or margarine. Whisk in flour, salt and pepper. Add the milk all at once, whisking till smooth. Cook and stir till mixture is thickened and bubbly. Stir in the blended vegetable mixture. Cook and stir till soup is heated through. Season with additional salt and pepper to taste.