In a large (10- to 12-inch) saute pan, cook the butter and onions on medium-low heat for 12 to 15 minutes, until the onions are lightly browned. Don't rush this step; it makes all the difference when the onions are well cooked.
Sprinkle the flour into the pan, whisk in, then add the salt and pepper. Cook for 2 to 3 minutes. Add the hot chicken stock and Cognac, and cook uncovered for 4 to 5 minutes, until thickened. Add the cream, if desired, and serve.