Combine the flour and butter in a medium bowl.
Heat a large saucepan over high heat. Add 2 cups stock; bring to a boil. Cook until reduced to 1/2 cup (about 5 minutes). Add the remaining 3 cups stock; bring to a boil.
Stir 1 cup stock into flour mixture, stirring with a whisk until smooth. Pour mixture into saucepan; reduce heat to medium-low, and cook until thickened, about 5 minutes. Stir in sage, salt, and black pepper.