In a large saucepan, add the stock, onions, carrots, celery, and bouquet garni. Bring the liquid up to a boil and reduce to a simmer. Simmer until the liquid reduces by half, about 1 1/2 hours. Strain through a fine-mesh strainer and cool. Refrigerate overnight and remove any congealed fat from the surface. The reduction can be stored in the freezer in ice cube trays for 1 month.