Also from Mastering the Art of French Cooking, this quick brown sauce is great when time is short. It is really nothing more than stock thickened with cornstarch or arrowroot. As Julia says, it has no culinary interest at all unless the base (stock) is excellent.
You will need
A 4 cup saucepan
A wire whip or whisk
2 T cornstarch or arrowroot
2 cups of excellent stock
Optional: 1/4 cup Madeira, port, or cognac
Blend the cornstarch or arrowroot with 2 tablespoons of cold stock, then beat in the rest of the stock. Simmer for 5 minutes or until sauce has cleared and thickened slightly. Season to taste. If adding wine or cognac, simmer for 2-3 minutes until alcohol has evaporated. This sauce may be set aside and reheated when needed.