Put into a sauce-pan the butter, with the flour. Simmer them till of a fine brown color. Mince the onions and cold ham. Put them into the pan, with another piece of butter, and a very little broth or warm water. Skim the sauce well, and let it stew gently for twenty minutes. Before you serve it up, stir in the lemon-juice or vinegar, and mustard. This sauce is used chiefly for fresh pork, or white poultry.