Boil the reserved marinade and beef broth in a medium saucepan.

Reduce heat and simmer for 30 minutes.

Strain and discard the onion, garlic and herbs.

Return the liquid to the saucepan. Add mushrooms, onions and carrots.

Simmer until vegetables are tender.

Mix together flour and butter. Stir half of the mixture into the broth.

Stir, cooking and adding more flour/butter until the mixture is thickened and forms smooth gravy