A demi glace( pronounced demmy gloss) is a rich brown sauce made with beef or veal stock and something called espagnole(one of the French "mother" sauces..for ex...bechamel sauce is a mother sauce as is bearnaise). It takes LOTS of work to do and as I've never bought any(nor made any) I'm not sure where you could get it in your local grocery. Possibly in the international section under France??)