Scrape juices and browned bits from reserved roasting pan (saute' pan) into large glass measuring cup. Spoon off fat, reserving 3 tbsp. (this is not necessary if using beef redi-base for low fat gravy).
Heat reserved 3 tbsp. fat (or 3 tbsp. butter if no drippings) in heavy large skillet over medium heat. Add mushrooms (reserving approximately 1/3), garlic, and onion.
Saute' until mushrooms are tender (have rendered their water and start to brown), about 6 minutes.
Transfer mushrooms and onion to bowl and set aside.
Add wine to skillet (medium heat). Boil until reduced to half, about 3 minutes. Add reserved (1/3) chopped mushrooms, chicken stock and degreased pan juices (or 1/2 tsp. beef redi-base). Bring to boil. Simmer at medium heat to reduce slightly, about 10 minutes.
Add cream and mushroom/onion mixture to skillet.
Mix 1 to 2 tbsp. water and cornstarch in small bowl until smooth. Whisk into gravy consistency and add to gravy.
Continue to simmer until reduced to desired consistency, whisking occasionally, about 5 minutes.
Season gravy to taste with salt and pepper.