Incorporate roux into stock and bring to a simmer. Add mushroom trimmings and bay leaf.
Simmer 30 to 60 minutes, skimming impurities as they rise to the surface of the sauce. Strain sauce through cheese cloth. Season with salt and pepper to taste.
*Note: A roux is a thickening agent commonly used in sauces made by blending equal quantities of white all-purpose flour with clarified butter or oil. To make a white roux, melt the fat over medium heat and add the flour, stirring to combine until chalky colored. It may be used now, or cooled and stored for later use.