Put the onion and butter in a saucepan on the fire. Let them both become brown; then add the flour, and brown that also. Stir all the time. Add the beef or brown stock, and cook until the sauce is a little thickened. Season with pepper and salt. Strain it to remove the onion.
A sauce poivrade is made by adding to the brown sauce, at the same time that the stock is put in, a cupful of claret, two cloves, a bay-leaf, a little thyme and parsley.
In place of claret, a teaspoonful of mustard, the juice of half a lemon, and a teaspoonful of tarragon vinegar gives a Robert sauce.