Sprinkle in all-purpose flour and stir to form a paste. Cook for about 3 minutes, stirring frequently until the flour is lightly browned.
3
Whisk in 3 cups of the unsalted beef stock.
4
Bring to a boil over medium-high heat. Then lower the heat to a simmer, add the sachet, and reduce for about 20 minutes, or until the total volume has reduced by about a third.