Lightly spoon the flour into a dry measuring cup, and level with a knife. Combine flour and 1/2 cup milk in a large saucepan over medium heat, stirring with a whisk until mixture is smooth. Stir in 1 3/4 cups milk. Cook for 4 minutes or until mixture is thick, stirring constantly with a whisk.
Add peas, sage, butter, and chicken, stirring to combine. Cook 2 minutes or until thoroughly heated. Remove from heat; stir in juice and pepper. Garnish with sage sprigs, if desired.