Combine first 3 ingredients in a large saucepan; bring to a boil. Cover, reduce heat, and simmer 10 minutes. Place half of broccoli mixture in a blender; process until smooth. Pour broccoli mixture into a bowl. Repeat procedure with remaining broccoli mixture. Return pureed mixture to pan.
Combine milk and flour, stirring with a whisk until well blended; add to broccoli mixture. Cook over medium heat 3 minutes or until mixture begins to thicken, stirring frequently. Add lemon juice, butter, salt, pepper, and mace, stirring until butter melts.