Place duck breasts, one at a time, in a heavy-duty zipper lock bag.
Using a mallet, pan, rolling pin, etc. lightly pound the duck until about 1/4-inch thick.
Season duck with salt and pepper.
Sauté onion and garlic in butter in a large skillet over medium heat for 5 - 6 minutes until soft.
Add duck and cook each side for 2 minutes. Remove pan from heat.
Combine remaining ingredients except heavy cream and slowly stir into pan while pan is away from the heat. CAUTION: BRANDY MAY IGNITE! Carefully return pan to the heat and cook for 2 minutes more.
Remove duck from pan when it is cooked to medium-rare temperature.
Continue cooking other ingredients until mushrooms are soft and just cooked.
Stir in cream, increase heat to high and stir sauce until thickened
To serve, arrange duck breasts on plate and spoon mushrooms and sauce over.