Cook the onions in the 3 tablespoons butter in a skillet over medium heat for a few minutes. When the onions are brown, turn off the burner temporarily so you won't ignite your kitchen. Or yourself.
Pour in the whiskey. As soon as it evaporates, turn the burner on medium-high heat and pour in the beef stock. Add salt and freshly ground black pepper to taste, then allow the mixture to bubble up and reduce by half.
Whisk in the remaining tablespoon of butter, then reduce the heat to low. Whisk in 1/4 cup to 1/2 cup cream, according to your taste. Allow the sauce to simmer and thicken for a few minutes.