Chateaubriand sauce is a sauce that was developed by Auguste Escoffier to accompany Chateaubriand steak.
It is made with chopped shallots, thyme, and bayleaf, mushrooms, and white wine, which is reduced, and then veal gravy is added, and reduced again. The sauce is then strained and finished with maître d'hotel butter and tarragon.
Escoffier's recipe is as follows:
CHÂTEAUBRIAND SAUCE
Source: A Guide to Modern Cookery; Auguste Escoffier, 1907
Put one oz. of chopped shallots, a sprig of thyme and a bit of bay, one oz. of mushroom parings, and one-quarter pint of white wine into a stewpan. Reduce the wine almost entirely, add one-half pint of veal gravy, and reduce again until the liquid only measures one-quarter pint. Strain through muslin, and finish the sauce away from the fire with four oz. of butter "Maître d'Hotel" (No. 150), to which may be added a little chopped tarragon. Serve with grilled fillet of beef, otherwise "Châteaubriand".