Melt 2 tablespoons butter in a medium skillet over medium-high heat.
Add mushrooms and saute about 5 minutes. (If using chanterelles, dry saute first for 1 – 2 minutes and let the mushrooms cook in their own juices before adding the butter.)
Add cognac, bring to a boil, and reduce liquid by ½, about 3 – 4 minutes.
Lower heat to medium; add cream, and simmer 5 minutes.
Remove skillet from heat and set aside.
Bring stock to simmer in a saucepan.
In a deep, heavy, medium sized saucepan, heat oil and remaining butter.
Add shallots or onions and cook until soft, about 2 minutes.
Add garlic and cook another 30 seconds to 1 minute.